The first NBA Jam game had a few hidden characters, but it has nothing on the Tournament Edition of the game, which went way over the top.

For instance: if you enter your initials as GRM

And your birthday as OCT 31

Your player turns into the Grim Reaper! Who looks kinda funny rather than scary.

Just be aware that this doesn't work on all revisions of the game, just v.1 and v.2.