Board Thread:General Discussion/@comment-26173936-20150328173404/@comment-8161155-20150329001200

My theory (WARNNG: My theories are often Bad):

Bill is the combination of the souls of everyone in the wheel. This is why he is so "insane", having ten personalities and all, and why he "gets really mad when he's mad"- whatever emotion he feels, he's got ten personalities feeling that emotion, so it adds up to a lot. If you shak hands with Bill, you "agree" to become part of him, so Dipper and Gideon are already doomed. Throughout the series, Bill will shake hands with everyone else in the wheel at some point, and at the end of the series everyone's souls will merge into a new Bill.