Here is the guide to get all secrete characters & levels.

Characters:

Black & Blue 1 - In challenge mode, get the ballsy trophy on the final qualifier course (extreme).

Black & Blue 2 - In challenge mode, get the ballsy trophy on the final qualifier course (extreme).

Chef Mutten - Complete Wipeout Zone within 1 minute.

Grasshopper - Play a multiplayer game.

John Anderson - Play a single player game.

Mad Cowgirl - Play a multiplayer game.

Most likely to succeed (second outfit) - Play a single player game.

Valley Girl (Second Outfit) - Play a multiplayer game.

Levels:

Easy Qualifier 4, 5, & 6 - Beat Easy Qualifier with a bronze trophy or better.

Challenging Qualifier 1 & 2 - Beat Easy Qualifier 4, 5, & 6 with a bronze trophy or better.

Extreme Qualifier - Beat Challenging Qualifier 1 & 2 with a bronze trophy or better.

Challenging Wipeout Zone - Beat Easy Wipeout Zone with a bronze trophy or better.

Extreme Wipeout Zone - Beat Challenging Wipeout Zone with a bronze trophy or better.

Happy Speedrunning :)