Well, I'll try. Haven't touched him in a while but here goes.
Story-wise, Mask was a knight and black mage who once served Ovelia before she got kidnapped and ended up with Delita. However, he left her service after an argument and became a wandering sword-for-hire. The tie-in with Ramza's story would be that there would be a bounty on his head, a mistaken identity case, and he'd clash with Ramza and co. until the real criminal showed up and then Mask would join.
Appearance...he's of average height, very thin. He has blonde hair, short and spiked, red eyes and, strangely for the universe this game is in, dark tan skin. He wears lighter armor then most knights and carries a sword called DemonBane. The black magic came from his mother, who taught it to him.
Personality...he's friendly, kind and gentle. He dislieks fighting but puts loyalty (and later, determination) ahead of anything else. He is not above bending rules to get ahead and to help others/himself. He is also (insanely) determined when fighting and will often fight past when it's good to do so.
I never really paired him with anyone, though I did try with Agrias once.