If you solve all 50 A puzzles, the game will give you the code to visit "Another World" and will unlock the evil sorcerer.
In Puzzle Mode, Select 1-A, then 2-A, all the way to 5-A. After beating the last puzzle in 5-A, you'll get the code and the end credits. After the DS restarts, you'll be able to play as the Evil Sorcerer.
Once you beat all the A worlds in the light world with DEV enter this code at that title screen: A Left Right A
Push Select, A, B, Left, Right, A, Select, Right in the main menu; this opens Sound Test