We've got our staple of Jewish superheroes. The Thing. Kitty Pryde. The Hebrew Hammer.
For those who are not familiar with the series, Captain Underpants is a children's superhero whose costume consists of... underpants. And a red cape. Can't forget the cape.
So, what makes the Captain possibly Jewish? Well, in one of the books, he is supposed to marry Miss Ribble (AKA Wicked Wedgie Woman). The ceremony is conducted by a rabbi. Underpants, in his regular guise as Benny Krupp, is shown wearing a yarmulke.
So, since that's the only instance to hint at the Captain's religion, is that enough for a Jew verdict? Let's analyze.
Wearing a yarmulke at a wedding does not necessarily make one Jewish (see "Goodfellas": Henry Hill wears a yarmulke. He is not Jewish; his wife-to-be, Karen, is). So, in theory, it might be the Wicked Wedgie Woman who is a Jew here. "Tara Ribble" sure doesn't sound Jewish, but there's no evidence either way.
And, speaking of names, the Captain's real name is the only other thing we can analyze here. "Benjamin" is Jewish enough (although obviously not exclusive), but the last name "Krupp" is clearly German, not Jewish (hockey player Uwe Krupp comes to mind). So the most the Captain could be as half Jewish, on the mother's side. Unless his father converted, of course.
But, on the other hand, he IS called Captain Underpants and that's just too cool to deny completely.