^^^ to this post... Lars? lol. Him and Kilelrbee are the only 2 characters your dont unlock through the 7 chapters... but you can through the Fragments Chapter and SP's... Havnt heard of any secret characters and I have all the ???'s filled.

*

EDIT:

Unlock Pain in Naruto Shippuden: Ultimate Ninja Storm 2Unlock through story mode.Unlock Lars in Naruto Shippuden: Ultimate Ninja Storm 2Earn 600,000 SP.Unlock Killer Bee in Naruto Shippuden: Ultimate Ninja Storm 2Earn 540,000 SP and Finish 37 Dolls Quest. Next complete "Fragments".

*

Found this on a site claiming to be the secret playables. Their not really secret though.