There are five secret characters that can be unlocked for play. The four developer characters cannot be unlocked playing as Max.
Grannie - Collect 40 water sprites and 2000 golden diamonds in the normal stages.
Corbie Dillard - Hidden at the top of the level right at the start of Ghost Stage 4-4.
Jools Watsham - Hidden above the house at the beginning of Ghost Stage 1-1.
Matthew Gambrell - Vertical boost above the checkpoint in Ghost Stage 2-1.
Troupe Gammage - Vertical boost above the starting tunnel in Ghost Stage 3-1.