Chase
To unlock Chase,defeat Mala Mala Jong in the last level of Adventure mode

Jack
To fight the Jack Spicer Robot you must get at least halfway through the game . Defeat it to unlock Jack.

Alternate costumes/outfits
Level up the rank from Grass Hopper in Training mode to Wudai to unlock new costumes. For some ranks it will unlock new costumes:

Monk costume
Robot costume
Street costume
Showdown costume

PS2 Cheats for Xiaolin Showdown

Xiaolin Showdown cheat codes

Xiaolin Showdown cheats