Companion

Early in the game, you will only have one companion, Duke.

As the game goes on, more slots open up, and more companion choices. Eventually, you have a main team of 4 companions (and possibly up to 4 alternate companions), and they will comprise much of your team's OA (overall strength).

Basics
Each companion has a class, basic stats, a skill, sometimes a special ability (e.g., control immunity), and, most importantly, a star rating.

There are over 60 companions, but many of them are not viable beyond the early stages of the game, or not generally available. You can see details on 59 of them in the Handbook.

You can have up to 8 companions on your team (only 4 in each formation). All other available companions are in the Tavern, where you can buy them for your team.

Both the Handbook and the Tavern can be reached via the NPC Shelly (at the left edge of the city), or from the Exploration interface (button at the top of the screen).

Stars
A companion can have 0 stars, 1 to 4 silver stars, or 1 to 4 gold stars. 1 gold star is effectively like 5 silver stars, 2 gold is like 6 silver, and so on, so the companions are ranked from 0 to 8.

The star rating determines:
 * Maximum stats at each level
 * Maximum evolution tier
 * How many alchemy crystals the companion can equip

It's also a general indicator of how good the companion is. In general, 3-gold and above are super-comps only available to big spenders; 2-gold are solid end-game companions; 1-gold can be pushed to the end if necessary; silver companions will become obsolete. However, not all companions are equal; Ying Chen and Sinclair may both be 1-gold explore companions, but only the former is part of many of the top players' teams at level 100.

It's also a general indicator of the price; most companions at the same star level cost the same amount.

Gaining companions
There are four kind of companions: reputation, mercenary, event, and exploration. Each is acquired in a different way.

In some cases, rather than acquiring a companion directly, you acquire a contract. This goes into your inventory; you can then double-click it, and the companion will become available.

When a new companion becomes available, you will get a popup in the lower left corner of the screen. You can click that, or you can go to the tavern (either via the NPC Shelly, or via the "Explore" button in the routine points window) at any point to look at available companions.

In the tavern, each companion will have a cost in gold, diamonds, or neither that you will have to pay to add the companion to your team. You will also have to have room in your team. After adding the companion, make sure to go to Formation and activate them.

Reputation
Reputation companions are usually acquired by a story quests, requiring you to complete a normal instances. Some are instead acquired by getting their contract from an elite chapter star chest (usually the next-to-last chest).

Most reputation companions cost gold to pick up in the tavern. If you later dismiss them, you will have to pay the gold again.

In general, reputation comps are not as powerful as other comps at the same star level.

Mercenaries
Mercenaries are acquired by defeating a specific elite (sometimes the mercenary himself).

Mercenaries cost diamonds instead of gold to pick up in the tavern, with the exception of Pathonia.

Event
There are three "VIP comps", who are treated as event comps, but are actually rewards for VIP 1 (Mr Lee), VIP 4 (Deborah), and VIP 6 (Kasumi).

All other event comps are given as rewards for events. Other than a few special cases (like Bobo, who was awarded to the player who spent the most diamonds during a 2-week period), generally these events require you to collect a certain number of something. For example, Tornado required collecting normal gear components; Lesley required collecting special items that were given out by another event.

Most commonly, they require Elder Scrolls, which (at least since January 2014) are only obtained by buying Chests of Mars for diamonds: 10 scrolls for 1-gold, 20 for 2-gold, and 80 or 100 for 3-gold.

Event comps are free in the Tavern once acquired.

Exploration
In Exploration, you can try to find the companion you want buy using the Rumor button (once free/day, after that it costs diamonds). Each exploration comp also has a minimum level you have to reach before he will show up in rumors. Once you've located the companion, you click Explore (3 times free/day, after that 20 diamonds).

Silver exploration comps are recruited by exploring for them until you find their contract. The more stars they have, the lower the chance of finding them on each explore.

Gold comps can be recruited the same way, but the chances are very slim. Instead, you usually acquire them through keepsakes.

Each gold explore comp has 5 keepsakes. If you find all of them, you get the contract. These drop much more often than the contracts, but still not very commonly (especially for high-star comps). It's also possible—in fact, likely—that you will get the same keepsake multiple times, and extras are useless.

When you don't find either the contract or a keepsake, you may find a lore chest, which has 20-500 lore and/or some other bonus, but usually you will get 20 lore (or 0, for free explores!) plus a small amount of gold, honor, or rep. Either way, this lore can be used to buy keepsakes.

So, for example, if you've exploring for Ying Chen, and you've found 3 boots, 2 razors, and 1 grindstone, but no armor or mantle, but you have 17700 lore, you can buy the armor and the mantle with that lore, and then use the keepsakes to get the contract.

Explore comps are free in the tavern once acquired.

Cost comparison
Since Merc, Event, and Explore comps are bought with different currencies (diamonds, elder scrolls, and lore), it's not easy to compare them directly, but here's an attempt.

It's generally acknowledged that the chance of getting an Elder Scroll from a Chest of Mars is about 1/20, and each Chest costs 50 diamonds, so each Elder Scroll costs about 1000 diamonds on average (subject to luck, of course).

In exploration, a 1* comp takes 26230 lore; a 2* takes 53143 lore; a 3* takes 123545. We could try to work out how many diamonds that is, but without knowing what the drop rates are for bonus lore, free keepsakes, and entire contracts, the best we can do is go by what people say they spend on these comps. If Kincade typically takes about 100000 diamonds, then each lore point takes 0.81 diamonds.

(We can ignore the fact that Sinclair takes an extra 1000 lore, and Ink takes 20 fewer elder scrolls; if you actually care about those comps, adjust for yourself.)

The obvious thing to notice here is that 1* and 2* explore comps, and 2* mercs, are horribly overpriced.

Of course some comps (Ying Chen, most obviously, but also probably Rosa) are overpowered for their star rating, so they might be worth overpaying for. But otherwise—well, instead of spending money exploring Guruk, it's probably a lot smarter to just save up for the next Frelia event.

Tips
Early on, it's tempting to change companions every time you get a new one. This wastes a lot of resources. All the gold, honor, evolution stones, etc. you put into a comp who's going to be replaced in a week could have been spent on another comp who sticks around longer.

In the midgame, it can be worth keeping companions on the bench to pull out for different fights. For example, there are dungeon levels full of physical monsters; if you call Ada out of retirement to replace Kelvin, you can get through them earlier.

If you're on a budget, don't spend diamonds on any explore comps but Ying Chen (and maybe Kincade, if you're playing for the long game). Save your diamonds early on, and use them to buy Ying Chen faster, or to buy someone like Frelia or Rosa. If you have a bit to spend, it's probably worth getting whichever of Kitty, Calvert, Hodge, or (maybe?) Neza matches your main class, and possibly another 1-gold comp along with Ying Chen before you can get Frelia or Rosa.

In the endgame, the only free comps you have to choose from are Kelvin, Martina, Andre, Bareth, and Lulu. Trying to make it all the way through the game with just four of those five will be difficult. You're going to need at least one better comp; Ying Chen, Lesley, Fredela, Frelia, and Rosa are probably the most popular choices.

0-star

 * Duke (Guardian, reputation, level 4): Duke is, of course, the best comp in the game, but replacing him is the emotional center point of the game, so you should do so as soon as possible.

1-silver

 * Valdes (Berserker, reputation, level 9): Surprisingly good for an early comp. Most people replace him long before he's outlived his usefulness.

2-silver

 * Anne (Mage, reputation, level 25): Another comp many people replace way too early. Most of the early monsters, and even more so most people you face in PvP, will have terrible magic defense. It's going to be a while before you get Ada—and, even when you do, it can be helpful to have two magic users on the team.
 * Lance (Swordsman, reputation, level 27): Swordsmen are almost as good at tanking as Guardians, and can also do some damage. Keep him around until you get a better tank, or stop using them.
 * Leo (Priest, reputation, 9 stars in chapter 2): Like most priests, Leo heals a fixed amount that very quickly becomes obsolete. The only reason to use Leo is as a gimmick (e.g., if you want to do 0 damage against the World Boss, use Might, with Leo in slot 1 and your main in slot 2).
 * Relic (Assassin, exploration, level 10): Relic may be an assassin, but she doesn't have high critical, back-row attacks, or control attacks. She does at least give you your first damage-over-time effect. Unless your team is short on physical defense.
 * Ankha (Guardian, exploration, level 20): A big improvement over Duke in stats, but he still does almost no damage, and you're better off with a tank who can fight, like Lance.

3-silver

 * Avril (Priest, reputation, level 48): Avril is more of a druid than a priest, but she was added to the game before the druid class existed. Her attack doesn't do much damage, and tranquilizing by 5 is usually not enough to make a difference. If you're going for early skill blasts this early in the game, her team-enrage-25 skill could be useful, but few people are. So most people skip her.
 * Neil (Guardian, reputation, 12 stars in chapter 3): An even better Ankha—meaning he's still useless.
 * Alisa (Hunter, reputation, level 42): Alisa could be useful if it were easier to build dodge this early in the game, but it's not.
 * Mr Lee (Monk, event, VIP 1): Not bad, but it's too hard to build skill this early.
 * Diana (Swordsman, event): I've never seen anyone using her, so it's hard to comment.
 * Pathonia (Assassin, mercenary, elite Anne level 24, free): The first real assassin, Path is useful for getting around guardians and other tanks, and this early in the game it's easy to keep her alive by hiding her behind Lance or your main.
 * Daisy (Mage, mercenary, elite Newman level 33, 2250 diamonds): Not bad, but Tarelg is almost as good now, and takes longer to become obsolete, and cheaper.
 * Tarelg (Mage, exploration, level 20): If you get him for free, he can replace Anne, or go with a double-magic setup, but don't spend a lot trying to get him.
 * Lady Kitty (Berserker, exploration, level 20): If you're a zerk, she's worth getting and keeping until you get Kelvin. A few people keep her even longer, going for a triple-zerk super-spec-sharing build. If you're not a zerk, if you get her for free, use her; if not, don't spend a lot trying.

4-silver

 * Edens (Assassin, reputation, 12 stars in chapter 4): A step down from Pathonia. She's only useful as a (phys-damage) glass cannon; you don't need one of those until later, and by the time you do, she's not good enough.
 * Ada (Farseer, reputation, level 50): A huge jump over Anne, Tarelg, or Daisy, you definitely want her, until you can replace her with Martina. In fact, she's worth keeping on the bench for a while after that.
 * Joseph (Guardian, mercenary, elite Garland level 43, 6000 diamonds): A huge step up from the earlier guardians, but still not worth it.
 * Ysuria (Priest, mercenary, elite Vela level 40, 6000 diamonds): If you're already building a skill-blasting team, having an enrage comp this far ahead of Lulu could make a huge difference. And you can already explore for Ying Chen at level 40, so it's possible.
 * Calvert (Swordsman, exploration, level 20): If you're a swordsman, you want him. Even if you're not, he's pretty handy, but don't spend too much.
 * Neza (Druid, exploration, level 50): Level 50 is pretty late to be getting a silver comp. Hopefully that's a bug they'll fix later, because Neza is clearly the druid equivalent of Kitty, Calvert, and Hodge.
 * Hodge (Hunter, exploration, level 20): If you're a hunter, you want him. If not, Calvert is probably a better bet, but if you get him for free, he's not bad.
 * Tornado (Swordsman, event, essentially free): Inferior to Calvert, a passable alternative if you don't have him, or an option for a triple-swordy super-spec-sharing build.
 * Deborah (Priest, event, VIP 4): Her healing becomes obsolete pretty quickly; after that, enrage 5 isn't enough to make a difference, so she's probably not worth using.

1-gold

 * Andre (Monk, reputation, 12 stars in chapter 5): First solid skill team-blast, with damage over time, and, unlike other monks, can be built up into a decent mini-tank. He'll eventually become obsolete, but he's a big help when you get him, and even more once you get a level-8 Havoc on him.
 * Martina (Mage, reputation, level 63): Solid magic damage to take out dodging hunters or skill-immune monsters, and all of the better magic attackers are expensive. Like most mages, she's very fragile, especially to skill attacks, but if you put enough resources into either defenses or dodging, you can keep using her pretty late.
 * Kelvin (Berserker, reputation, level 65): Every zerk needs Kelvin for spec-sharing; every non-zerk needs Kelvin for the only early counter worth anything. Like Martina, his main problem is dying too fast. His skill attack is also abysmally useless. But keep him until you buy someone better.
 * Sagna (Swordsman, mercenary, elite Blake level 50, 13500 diamonds): A solid swordy, but not worth the price unless you're in the narrow range of people who want to buy comps but can't afford a better one.
 * Freya (Hunter, mercenary, elite Magellan level 52, 13500 diamonds): A solid hunter, and charm plus reduce control resistance should help a lot for spamming, but not worth the price.
 * Mr Tang (Berserker, mercenary, elite Wiese level 55, 13500 diamonds): A solid zerk, but Mr Chen is only about 20000, and Kelvin is free.
 * Sinclair (Assassin, exploration, level 40): Not bad, but if you want an assassin, Fredela is cheaper, and besides, your lore should be going to Ying Chen.
 * Ying Chen (Monk, exploration, level 40): A game changer, possibly even a game ruiner. Much better damage than Andre, and he also de-rages the energy team so they can't retaliate. Eventually, 80% of the people you compete with are going to have Ying Chen with a Havoc 10 and lots of formation speed, and if you don't have Ying Chen (or someone better like Rosa) with a Havoc 10 and even more formation speed, you can't win by skill blasts. The only reason to skip him is if you're saving for Rosa instead, or you've decided not to go for skill blasting.
 * Agrim (Guardian, event, supercheap): An amazingly good guardian; too bad guardians suck, especially when people start getting monks and assassins.
 * Lesley (Druid, event, essentially free): The only healer cheaper than Luna whose healing is a percentage rather than a fixed amount, so she never becomes obsolete. She can also be a decent mini-tank. Unfortunately, her damage sucks. But as a support character, she can really help, especially for a zerk or swordy who wants to win through counters.
 * Fredela (Assassin, event, 10 elder scrolls=10000 diamonds): A bit of a specialized character, but if you need a kick-ass physical glass cannon—e.g., to knock out the opponent's havoc-10 Ying Chen (or his Lulu if he's only using havoc 8)—Fred is by far the cheapest option.
 * Moe (Guardian, event, cheap): Inferior to Agrim for more money (and sold less often).

2-gold

 * Bareth (Hunter, reputation, level 76): Every hunter should use him, unless you're planning to buy Kincade; everyone else should at least consider it. He's weak at first, you can build him up to do some serious skill damage and halfway decent physical damage. And, unlike a monk, he can stay alive through dodging to skill again. And his self-enrage means that he skills again one round before everyone else.
 * Lulu (Priest, reputation, 15 stars in chapter 6): Her healing is almost obsolete from the moment you get her, but enrage 25 is huge until you have level-10 havocs, and huge again once people start deraging. Plus, she makes a pretty good mini-tank.
 * Rosa (Farseer, mercenary, elite Gustavo level 70, 37500 diamonds): The best of Ying Chen and Martina in one, plus she can survive a lot better than either of them. What's not to like? Other than the price, which is out of range for most people except those who can afford the 3-gold super-comps… Still, a mid-spending player, or even a cheap one, can eventually save up 37500 diamonds, especially if he skips Ying Chen.
 * Ithenheart (Assassin, mercenary, elite Brazor level 60, 37500 diamonds): The best assassin in the game… but does anyone need an assassin in this price range?
 * Amaranth (Druid, exploration, level 50): More of a farseer than a druid, since he doesn't heal or otherwise buff your team, and his debuff is useless. Not bad, but Rosa is better and cheaper.
 * Nurgle (Guardian, exploration, level 50): A great guardian, and he also can give your whole team control immunity. If you're worried about people stealing your top-10 slot with a charm aeon or comp and a bit of luck, he might be worth using while you're AFK, but for anyone else, that's a lot of money for so little offense and support.
 * Winston (Swordsman, exploration, level 50): His derage abilities mean you can use him to preempt skill blasters even if they can't be one-shot, and unlike an assassin he also survives forever, and can be built to counter as well. The only real worthy alternative to Rosa in the same price range, but as with all the 2-gold merc and explore comps, that's a narrow price range—and in this case, Dante is not much more than twice the price and he's a lot more than twice the comp.
 * Guruk (Farseer, exploration, level 50): He'd make a great replacement for Martina, but Rosa and Frelia both make even better ones.
 * Bunny (Monk, event, expensive): Better skill damage than Ying Chen, and easier to keep alive, but without the derage he's not a replacement, and how many skill blasters do you need? You're eventually going to need some sustained physical or magical damage too…
 * Mr Chen (Berserker, event, 20 elder scrolls=20000 diamonds): A great drop-in replacement for Kelvin for zerk spec-sharing, but the price is pretty high, and it may hard to find room in your team for him.
 * Kasumi (Assassin, event, VIP 6): If you're going to go to VIP 6 eventually, get there as soon as possible and start building up Kas. When you first get her, she'll easily outclass most of the people and monsters you're fighting. And you can build her into an end-game character, although she's a little fragile. Also, charm always helps when you need to spam your way through tough PvE fights.
 * Frelia (Priest, event, 20 elder scrolls=20000 diamonds): An interesting combination of an enrage/heal skill and a non-rage-increasing multi-target normal attack, she's definitely worth the money. She can replace both Lulu (if you're going for one-shot skill blasting; not so much for sustained fighting) and Martina.

3-gold

 * Elaine (Mage, mercenary, 112500 diamonds): Kick-ass magic damage to multiple targets, plus charm. Maybe worth a look after Kincade, Dante, and Luna, but shouldn't be your first choice.
 * Kincade (Hunter, exploration, level 60): Kincade is what separates the top 10 from the rest of the field. Huge physical attack to all enemies, or even huger skill attack to all enemies, depending on who you're fighting.
 * Luna (Priest, exploration, level 70): If you've got diamonds to blow, replace Lulu with Luna. Healing that doesn't become obsolete, enrage 25, and debuff immunity (which includes control immunity, to prevent those pesky charmers from getting lucky and stealing your spot).
 * Dante (Swordsman, event, 100 elder scrolls=100000 diamonds): After Kincade, the next biggest game-changer; Dante is impossible to kill, and hard to survive.
 * Freja (Mage, event, 100 elder scrolls=100000 diamonds): Seems inferior to Elaine, for not much cheaper.
 * Rexx (Monk, event, about 100000 diamonds?): Repeated super skill blasts instead of just one, and not fragile like most monks. Maybe a non-hunter could use him in place of Kincade (to share specs with YC instead of with Bareth or nobody), but nobody seems to have tried it.
 * Ink (Mage, event, 80 elder scrolls=80000 diamonds): He seems like a perfect counter to Rexx, who nobody uses; otherwise, almost any of the other 3-star and even 2-star magic users are better.
 * Ursula (Farseer, event, hasn't been available since 2013): All those 50% chances for each of her side effects seems like they'd make her too risky compared to Elaine or Freja. Since you can't get her anyway, it hardly matters.
 * Greenbeard (Druid, event, not yet available): Skill immunity is even better than derage. And with 20% more INT than any other comp, his damage is probably pretty good. Hard to say until he's available.
 * Gabriel (???, none of the above, more CRC reward coins than it's possible to have): ???

4-gold

 * Bobo I Touchdown (Berserker, event, spend the most diamonds over a 2-week period): If your name is one letter away from Bobo and you play that American sport for a living—baseball, that's the one where you touchdown the puck, right—then enjoy the gift from China. For anyone else, I don't think you want to try to outspend the rest of the game for a comp, unless you're buying the rest of your team at the same time.