Companions are one of the most crucial parts of Star Wars: The Old Republic when it comes to everything but PvP and operations. Companions will follow you anywhere in the game as long as there is room in the group and it's not specifically a PvP zone (more on this later) and will fight by your side utilizing their speciality (usually either DPS, tank, or healer). They can be programmed to act in a few specific ways (like pets), but otherwise, they are ultimately just another party member in your group.
To understand a little bit more, think of them as persistant pets. The first one you unlock is a tank, in your infancy it will taunt and keep the attention of most enemies you're fighting in order to make sure you can focus on killing it. As you advance through the game, more specialized companions come with healing and DPS talents to compliment your main role. For instance, if you're a tank, you'll often want the DPS companion or the healing companion. If you're a healer, you'll want the DPS companion or the tank companion. Finally, if you're DPS, you'll want the healing companion or the tank companion. This all depends on how you've built your character out.
For instance, if you find yourself never losing HP, then you'd want the DPS companion, likewise, if you're having difficulty staying alive then you'd want to consider either the tanking companion or healing companion (try either to see how it goes).
Companions wear equipment just like you do, actually, the same equipment. They can act as either equipment storage or, alternatively, you can equip them with legacy gear (including with high level mods). Companion kits alter the appearance of your various companions and are optional.
Companions have stances, generally two (except for HK). This puts them into either a heal, DPS, or tanking role. Companions usually sort of have a main ability (that lines up with their skills) but can often use their alternative spec as well.
Healing companions have a CC ability that is channeled. Turn it off if you need additional healing out of them.
If your companion isn't doing well, check its gear. Make sure to keep its gear close to your level.
If your companion dies, right-click after battle to revive it. Additionally, companions can sell grey items for you (press 'N' to bring up the dialog box for more companion info and to assign missions).
Sometimes your companion will want to talk to you in private, either in your ship or a Cantina. This will usually impact their affection and romance options. It's skippable if you don't care about these things (which is totally fine).
Companions love gifts, just see our gift giving guide. Their affection will reward you with gifts and, depending on the companion, an optional romance segment (similar to Mass Effect). It also impacts the time they take to complete crew skills.
Companion's have crew skills, which will allow them to venture forth and gather or craft for you. You don't physically do the crafting, they do. Each companion gives you a bonus to your selected crew skill depending on the companion.
You can only have so many players in a party. When more than 2 players are in a party, one companion unsummons, and when the party is full there are no more companions. They will not be summonable in an operation group, nor will they appear in ANY PvP area.
To find out more about each specific companion, including what gifts they want and what their roles are and crew skill bonuses are, check out our individual companion guides:
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