The character who gets eaten by T.H.U.D. (the sewer monster) in the openening Neversoft logo movie is Eric Sparrow (from Story mode).
The game features three unlockable caracters. Gene Simmons of KISS, Iron Man, and a swamp creature called T.H.U.D.
An unlockable level, "Hotter Than Hell", can be unlocked by beating the Story Mode once on any difficulty. The level is a KISS concert surrounded by ramps, rails, and even a go-kart track with a go-kart you can drive. The letters K-I-S-S are floating around in the air by jumps (all on the side of the stage you start on), and if you jump into all of them (in no particular order), KISS will come on stage and play a song for you. You can run up onto the stage while they're playing, but if you accidentally walk into/skate into one of them, you will bail. You can also go get the go-kart while they are playing (you don't have to be near the stage at all times to hear the music) and jump it over the fence, then drive up onto the stage. Also, the fire on stage will not hurt you.