If I was your DM, I'd have you go through a skill challenge (Athletics, Insight, Intimidate, Nature, Perception). If you fail, it would attack you; if you succeed, it becomes a companion character under your limited control using the DMG2 rules. However, I'd first make sure it was OK with the rest of the players for two reasons. First, I wouldn't want the others to feel like you're hogging the spotlight with essentially two characters, and secondly, a companion character either means less XP for the other PCs or tougher encounters.