You can't costumize him in Astro Knights Island or Super Villain Island but you might search the common rooms for someone with his face.
They never show his face. It's purposefully hidden.
On Astro Knights Island in Poptropica, pretty much like classic knights (armor, cloaks, swords).
Like any normal person looks
Actually,he's not wearing a mask,he has face paint on his face,and i don't know what he looks like when its off.
They wore armour and helmets.
Heavy armored soldiers on horse.
They look like your face!!!
Nowadays much like anyone else's.
like a human's face
You can't "costumize" (copy) Mordred in the game, but you can find users in the multi-player room that have his "cyborg" face, and copy those.