Chaos

Chaos is a powerful water-like creature who is a mutated Chao and one of the many characters in Sonic the Hedgehog series that is the main antagonist of Sonic Adventure, but also a minor villain in his Perfect form as a boss in Sonic Generations.

History
Chaos was once a Chao, but became mutated by the power of the Chaos Emeralds. He became their guardian, as well as the guardian of the Chaos Emeralds. One day, an ancient tribe of Echidna discovered the shrine, and wished to use the power of emeralds to conquer the world. Tikal stood in their way, telling them what they were doing was wrong. They payed her no attention, and attacked the shrine. In the process, they stomped on all of Chao who lived there. This enraged Chaos, who used the powers of the Chaos Emeralds to destroy the entire Echidna civilization. Tikal used the power of the Master Emerald to seal Chaos, and herself away.

Sonic Adventure
Chaos also appeared in Sonic X anime as one of the main villains based on the video game. Essentially, the story was extremely identical and Chaos served the same role.

Adventure Mode
Knuckles, the last surviving member of the Echidnas, was placed in charge of guarding the master emerald on Angel Island. He didn't know why, but regardless he knew it was his duty. One day, while dozing off, he awoke to the sight of a shattered emerald, and a strange being, Chaos standing in front of him. Chaos got away, and Knuckles started his quest to repair the master emerald. Eggman was able to find Chaos, and made him his servant. As the story goes on, Eggman keeps feeding Chaos more and more emeralds, until Sonic makes it onto Eggman's flying fortress the Egg Carrier, and defeat Chaos, knocking out the six emeralds he had at that point. The Egg Carrier crashes, and Knuckles holds on to the emeralds, thus ending the main story.

Final Story
Knuckles takes back all of the Emerald Shards, and the six Chaos Emeralds back to Angel Island, sending it back into the sky. As Eggman plots his revenge, Chaos finds and attacks him. Eggman flies to Angel Island to get Knuckles' help, but Chaos followed him and made quick work out of both of them. Sonic goes to see what is happening, and Knuckles tells him about Chaos' return. Sonic and Tails look for the final chaos emerald in the tornado, but Chaos gets to it before them.

Using the power of the seven emeralds, as he did thousands of years ago, he becomes Perfect Chaos. He floods the city of Station Square, and starts attacking everything. Eggman, enraged that Chaos betrayed him, tries to use a new Egg Carrier to defeat him, but with a single blast Chaos rips the ship in two. Just when all seems lost, Tikal shows herself to Sonic, and tells him that they have to seal Chaos away before he could do any more damage. Sonic refuses, saying that if he is still angry it will just happen again. So, Sonic's friends gather the emeralds, and Tails tells Sonic that Chaos only used the negative powers of the emeralds, and that Sonic can still use the positive powers they posses. After this speech the emeralds have their colors returned, and Sonic becomes Super Sonic.

After Chaos is defeated, he calms down, and returns to his more friendly nature he had before the shrine was attacked. Pleased, Tikal thanks Sonic, and she and Chaos return into the master emerald.

Sonic Generations
Chaos reappears only as Perfect Chaos in the Station Square as a major boss from the classic Sonic game, Sonic Adventure. He starts to go on a rampage. Luckily, Sonic didn't need to use the Chaos Emeralds to become Super Sonic and defeat Perfect Chaos. Sonic rushes and speeds up to Perfect Chaos's body, hitting his brain several times. After that, Perfect Chaos was defeated and sunken back into the water.