Peter Riviera is an actor and performer. The team hires him to get on 3Jane's good side in the hopes that she'll invite him into Straylight so they can complete their mission. Why would 3Jane like him, you ask? Well, turns out he's also a drug fiend and a wee bit of a sociopath who "[c]an't get off sexually unless he knows he's betraying the object of desire" (7.106). He also uses his holographic modifications to try and get under people's skin by tricking them into seeing things that aren't real. So, yeah, we guess that's her type—sociopaths.
Naturally, Riviera betrays the team once inside and joins team 3Jane. He says he does so for the sake of "perversity" and the "enjoyment of a wholly gratuitous decision" (18.76). However, the way he talks about Wintermute's profile on him suggests something different. He refuses to be seen as a "statistical animal" and knows Wintermute is wrong about him (18.74). Combine this with his fondness of playing tricks on people, and one possibility suggests Riviera wants to have power over people but doesn't want to be controlled or labeled by them in return (or, for that matter, machines, AI, society, and so on).
Of course, this is only one of many ways to read this guy. You could also choose to write him off as a worthless jerk—it's totally your call. The major takeaway here is that you can't trust much of what Riviera says about either himself or other people. There's no truth to the man. You've got to read between the lines and see what you find.