Chaos

Chaos is a Chao mutated by the Chaos Emeralds that acts as a guardian for its species, protects the Master Emerald, and provides clear water around the altar of the Master Emerald. It is a water-like being that can easily manipulate its body, allowing it to stretch limbs or liquefy itself. Without any Chaos Emeralds, it is known as Chaos 0, but with each Emerald it absorbs, it transforms into a more powerful form named from the equivalent number of Emeralds, eventually becoming "Perfect Chaos" with all 7 Emeralds.

After the Echidna tribe attempts to steal the Chaos Emeralds, harming various Chao in the process, Chaos absorbs the negative energy of the Emeralds, becomes Perfect Chaos and causes chaos until being sealed inside the Master Emerald with Tikal. It is released thousands of years later in Sonic Adventure, so Dr. Eggman can use it to conquer the world. Chaos, still filled with negative energy, absorbs Chaos Emeralds given to it by Eggman, and after Eggman is defeated, Chaos absorbs the last emerald and becomes "Perfect Chaos".

It is defeated by Sonic who has absorbed the positive power of the chaos emeralds, which purifies Chaos, and allows it to leave peacefully with the spirit of Tikal. It reappears in Sonic Adventure 2 as a bonus character and in Sonic Battle as a playable character.