Nautilus

Nautilus is a character created by Ol666. He was originally an opponent in the levels Nautilus and First Crush, but eventually decided to go on the side of the player.

History
Nautilus appeared as one of Jestar Heart’s assassins in First Crush, where he tried to take Hal’s souls with Aura. Although the two of them failed, he later returned to attack by himself in Nautilus. After being taken over by The Core and being formed into N.A.T.U. in Party Time, Nautilus wasn’t seen again until Surface Tension, where he was the only assassin left alive after Medusa turned the rest to stone. He revealed that Medusa was created with a clone of his body, and after breaking out of the Heart Locket with Hal, Nautilus decided to be an ally and help to stop Medusa.

Nautilus reappeared in the level Medusa, near the end, to save Hal at the last second from a deadly attack. This was able to defeat Medusa, however, Medusa got away.

Appearance
Nautilus is a blue square with vertical eyes and square, clawed hands. His attacks usually focus around beams or moving around the screen.

N.A.T.U
N.A.T.U is a combination of Nautilus and The Core, where Nautilus is fully dependent on Core and Core is a main antagonist. It's a square covered with grey, metal plates. He also has a red, hexagonal eye.

Trivia

 * Nautilus’ eyes become rounded when he uses his laser attacks.
 * N.A.T.U. has no canonical meaning despite being an acronym.