Chuck may not be the most tactful but neither is Stan.  It figures that those two would come up with this theory.  But it’s a valid theory, in my opinion, and one no less likely than any other.

Hmm…. I wonder if I can get the ninja kid into a Pikachu costume…?

=Comic transcript=============================
Chuck: Stan, I have a theory.
Stan: What’s that?

Chuck: I think the fat ninja kid may actually be a Pokemon.
Stan: Why? Because he’s round and Japanese?

Chuck: Partially. But… Mostly because all he says is “I am ninja” or some portion of that phrase. Just like a Pokemon.
Stan: Maybe.  Are you sure he’s not just retarded?
Chuck: Not really, no.