One of the more natural places to explore mental illnesses in RP settings, wethinks would be the World of Darkness. There's one clan of vampire in there that every single member is insane in one or more ways. Among other things like social dysfunction, the character we're working on creating now has both suffered global amnesia and is schizophrenic. Because there's no pattern to the insanity in that setting, you can go with whatever you feel like, so long as the character is dysfunctional in some way.