User blog comment:27finnsprincess/Season Finale Predictions!/@comment-5425660-20120901134006/@comment-5242921-20120901134832

Jake knows Finn too well to fall for that without suspecting brainwashing or insanity, and PB is smart enough to sense when something's not quite right. (Her responses to Finn's actions in "The Real You" and "Go With Me" prove this.)

The only person susceptible to such actions without suspicion would have to be Flame Princess.

Or maybe Ice King. But Simon has "Wizard Eyes" and would know if Finn was possessed.