Phobos is your stereotypical massive 2-hander. While he's much smarter than he looks, you can tell just by looking at him that he's not gonna be using a gun.
Spoiler? To unlock Phobos, you have to do a few side quests during the first part of the game. He calls himself Beg and acts dumb to fool the guards, but reveals himself later to be smarter than he'd acted and becomes your companion.
Phobos may be the most level-headed companion you get. Not what you'd expect, but he's a natural leader and has a big heart for his people. He actually co-founded the Mutant Nation, and the reason he was in Ophir is he was trying to save his fellow mutants.
He also makes it clear that he has a huge disdain for mutants soul-bent on death, rather than peace with humans.
Phobos is one of the better combat companions. You can set him to go on Offense or Defense, but I prefer Offense. He has enough hit points that he's somewhat sustainable, and it's nice to have another decent damage-dealer on the team.
Phobos gives an extra +250 weight (which is ridiculous to me, considering it seems hard to reach the max anyways.)
If you earn his Ally bonus, he gives an extra +1 to Exploration. Probably the worst Ally bonus ever.
Phobos is involved in a few quests as Beg before he reveals his true self.