Get an "A" rank on all levels to unlock the original Kameo costume.

Break the indicated high score in an action level to unlock the corresponding bonus:

Big Kameo head: Thorn's Castle, 2,500,000 points
Big Troll heads: Thorn's Castle, 2,500,000 points
Boss battles: Water Temple, 2,500,000 points
FMV player: Forgotten Forest. 1,600,000 points
Kameo in her original purple costume: Thorn's Castle, 100,000 points

Successfully complete the indicated task to unlock the corresponding Elemental Sprite:

Ash: Rescue Halis
Major Rush: Defeat the Shadow Troll in Lobster Hollow
Pummel Weed: Defeat the Shadow Troll in the Enchanted Kingdom
Rubble: Defeat the Shadow Troll in the Forgotten Forest

Each Mystic Eye is located around the Enchanted Kingdom:

Eye One: Pummel Weed is required. Go to the beach by the palace and use Pummel Weed's Uproot attack and flip the shells. It is under one of them.

Eye Two: Ash is required. Go to the dungeon. Go straight through the first doorway and turn left. Light the torches on the wall and go in the room. It is floating in the middle of the room.

Eye Three: Chilla and Major Ruin are required. Go to the Throne Room. Go right into the Ancient Tower. Go left along the wall until you see a "quarter pipe". Use Major Ruin and go up to the second level. Go to the frozen wall and use Chilla to climb to the next level. The Eye is in the middle of the shiny light in the middle of the tower.

Eye Four: Thermite is required. Go to the Chamber of Living Portraits. Use Thermite's Mortar attack and blow up the Statue Of Thorn.