Successfully complete game 100% (all coins collected) with a character (eight pro skaters and one edit skater) to unlock a hidden skater. Highlight one of the default characters and press Circle to play as one of the following skaters. Each character has a different secret counterpart:

Arto Sarri: Gorilla
Chris Senn: Gurlukovich 2
Colin McKay: Solid Snake
Danny Way: Unknown
Edited Skater: Grey
Kerry Getz: Vampire Killer
Mark Appleyard: Raiden
Rick McCrank: Gurlukovich 1
Steve Williams: Frogger