You can acquire additional characters by meeting the corresponding requirements, as indicated below.

Coral Golem
Located behind the waterfall in Coral Cave

Cthulhu
Near the southeastern point on the map, look for him in a floating temple that you can access using the Dae-whale

Doctor Blood
Use the Dae-whale to enter a floating island on the map's northwestern corner, then defeat him once you find him inside

Nigiri-Chu
Defeat the Fish Force in the gladiator arena (which you will need the Dae-whale to access)

Ninja Z
Defeat Ninja Z after finding him in a room on the other side of a red teleporter located in the third tower (you'll have to look behind some scenery to find a hidden passage)