i know ive seen characters that were good and bad on a split personality level before, but the one from batman that came to mine in particular was more of a passive henchman/crazy villian boss kinda combo
in the animated series there's an english butler lookin guy who just stands around and carries/obeys a gangster-looking puppet who barks orders and tends to be rather aggressive. The english guy's second personality is played through the puppet (ventriliquist style), and so both himself and the other members of their group treat it as an actual person. Its weird, yet interesting.
But yeah, it would be cool to have a ninja like that...although i would imagine it turning into more of a Dr Jekyl/Mr Hide situation