No matter which profession you choose, BioWare's Star Wars: The Old Republic loads players up with colourful companion characters, ready to fill any hole that might come up. Not like that -- well, not like that officially.
You can keep your sexy Old Republic fan fiction to yourself (or mail it to me for critique), the companions BioWare has set up are there to share in your adventure, not your bodily fluids. They are fully-functional characters fitted with a sophisticated (but not too sophisticate) artificial intelligence, capable of fighting by your side through much of The Old Republic's more singleplayer-orientated content.
And they can join parties too.
Normally whatever companion you may be travelling with will disappear when you join a group of players, reappearing as soon as you disband. But players aren't nearly as reliable as artificial constructs, so when a party member has to leave in mid-adventure, the leader can choose to fill the spot with one of the remaining members' companion characters.
Mind you there will be certain situations where the limited AI and command bar for a companion just won't be enough, but in many cases you won't even know your real friends aren't there.
Companion Characters Update - Part 2 [The Old Republic]