Which character had the most baffling personality shift?S

Sometimes you grow to love a character — or at least get used to him or her having a particular personality. And then... all of a sudden, this once-familiar character has a completely different personality, in between movies or seasons of a TV show. What's the most baffling personality shift of all time?

Please include a picture or clip of the character in question. Thanks!

Top image: Captain Jack Harkness on Doctor Who.