All secret characters - At the "white" Konami screen, press Up(2), Down(2), Left, Right, Left, Right, B, A, B, A, Start to unlock all secret characters. If you entered the code correctly, you will hear a scream from Danny Way.
Secret characters - 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 X to play as one of the following skaters. Each character has a different secret counterpart: