Chaos

Chaos (カオス Kaosu) is one of the characters in Sonic Battle. It was trapped inside the Master Emerald for thousands of years before being released by Dr. Eggman in a plot to conquer the world. Chaos is the strongest playable character in the game (minus Emerl as he is customizable), but also by far the slowest.

In Sonic Battle
When Dr. Eggman threatens the Earth with his Death Egg in the final story, a nearby volcanic crater is flooded, and Chaos emerges. It can then be challenged by the player.