Every secret character
Just press Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, Circle. A scream from Danny Way will be heard to verify entry.

Kid mode
Finish every goal in practice mode.

Xtreme route levels mode
Finish every goal in arcade mode with a pro skater. Because then more goals will become available. Each pro skater has different goals.

Secret characters
Finish the game 100% (every coin collected) with a character (eight pro skaters and one edit skater) to have a secret skater available. Then highlight one of the usual characters and press Circle to play the game as one of the skaters listed below. The different characters have 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