Everytime I try to create a character with the name Avenger or Marvel in the name it gives me some error message. Not a name used message but something else. Yet I've seen leagues with the name Avengers and Marvel.

It may be different between character and league names. Been a while so I don't remember, though.

I believe they are though I haven't played in a couple of years. I remember seeing someone that made The Hulk, but I don't think he called him The Hulk but something close to it.