There are Hidden Characters located in several of the Ghost Zone stages. You'll need Grannie to be able to get to them. Once you rescue them just clear the stage and then you'll be able to choose them from the character select at the Title Screen (they all play the same as Grannie).
Character
How to unlock
Hidden Character 1
Ghost Zone 1-1. Boost straight up from the start to find him.
Hidden Character 2
Ghost Zone 2-1. Boost above the first Checkpoint to find him.
Hidden Character 3
Ghost Zone 3-1. Boost upwards as soon as you are out of the narrow path.
Hidden Character 4
Ghost Zone 4-4. Boost upwards at the start to find him.
Beat first 16 standard levels to unlock the jump pad to the moon levels.
Play as Brown Haired Guy
Enter the mirror in the main hub to access the spooky worlds, You Must First Unlock Grannie, then go to spooky world 3-1 and find the hidden character.
Play as Grannie
Collect all Water Sprites and Golden Diamonds in the normal worlds.
Power Ups
Unlock in Grannie's Attic by collecting gold diamonds.
