For me so far, it's probably my brain. I categorize all my aspects that make me up into characters that define that aspect or one I made up to do that. I shall list for your amusement what goes on in my brain.
Schism: I made this guy a while back to personify my calculated and cold approach to problems, my naturally deceptive nature, and my flamboyantly arrogant attitude.
Slaanesh: From the WH40K universe, Slaanesh is the god of depravity and other stuff of that sort, greed, gluttony, excess. He is my pleasure-seeking side and is a pretty big part of how I act. He keeps me making decisions for the best benefit to me or others.
Observer: From the TribeTwelve series on Youtube, he is my silent and watchful side always talking in tiny details nobody notices. He doesn't do much.
Rainbow Dash: After some wacky hypnosis shite, I got her as another persona. Basically my competitive and loyalty to my friends side
I could elaborate more but I do not want to bore the pants off anyone. But for other final bosses, the entire state of Indiana is a big final boss and pessimism is another of mine.