Grace Van Pelt. The entire series is based on the bad guy being somebody we've known from the beginning, and often is who we expect last. The most innocent seeming person. Here are some of my reasons. Lamest reason, she has red hair. Better reasons - She's the newest member of the CBI. (And Red John has already proven s/he can get into the CBI) Jane suspects she is hiding a 'deep emotional trauma'. Something she won't even admit to herself. Serial killers often have some kind of trauma, a messed up childhood or abusive ect, (at least in these cop shows I can't vouch for real life) and most often they create a world around them where the trauma did not exist. :D She is dating a killer at one point. I don't think that was accidental. She has Rigsby. If I were Red John, in the CBI, I'd want at least one gun who I knew could not shoot me no matter what. She's been purposely stringing along Rigsby too, blowing hot and cold in amazing ways. Unreasonable even for PMS. She is always challenging what Jane believes. She's the last person I'd expect, which makes her the most likely by the show's standards. Red John needs an in at the CBI so it makes sense. Red John WANTS to be close to Jane. Grace is always noticing things and holding back. Jane often calls her out to announce these things. It's like she's trying to hide how smart she is. Grace hesitates when Lisbon announces they are off the Red John Case. She also says that as a CBI agent they are quote 'fighting evil and injustice' something cop shows are famous for having serial killers disillusioned as. Grace also seems to try too hard to seem ignorant