Get the Dae-whale, then fly to the floating temple located at the southern-most area of the map, and defeat Cthulhu to unlock Cthulhu.
Get the Dae-whale, then fly to the gladiator arena, and defeat the Fish Force to unlock the Nigiri-Chu.
Use the red teleporter in the third tower to reach a room with a secret passage hidden by robotic scenery. Use the passage to find Ninja Z. Defeat Ninja Z to unlock him.