Jiraiya's son. He's definitely Jiraiya's son. I mean, I know it sounded ridiculous at first, but the guy summons frogs, has Fire Release and knows the Rasengan...and has white hair. Who else does that remind you of? Plus, I bet that if he takes off that mask he'll probably have those markings below his eyes too.
Not to mention, its not difficult to imagine Jiraiya sleeping with some random woman and getting her pregant, the guy was a massive pervert. Then he returns years later as a mysterious stranger and teaches the kid the Rasengan and lets him sign the toad contract. Naruto himself would have never noticed the name signed between Jiraiya's and his father's...or Minato's or himself depending on how old Kashin Koji really is. That name sounds like an alias anyway, the guy wears a mask and acts all mysterious. But the point is, Jiraiya can't stay around to raise a kid, he has a duty to his village, so he goes off and never returns. Then the kid grows up, puts on a mask, takes an alias and joins Kara.
