Chaos/History and appearances

Past
Chaos was originally a normal Chao, but prolonged exposure to the energy of the Master Emerald caused it to mutate into a strange water-like creature. It used its new powers to guarantee clean water around the altar of the Master Emerald, and to protect its fellow Chao. However, about four thousand years ago, the leader of the Echidna tribe, Pachacamac, attempted to steal the Emeralds for use against his enemies. Tikal, his daughter, resented this idea, worrying that it would upset Chaos and it would bring wrath to them, but Pachacamac did not want to hear it. His warriors stormed the shrine, where the Emeralds reside. Many Chao were injured in the process, as was Tikal herself. Chaos, in a fit of rage, absorbed the negative power of the seven Chaos Emeralds, and transformed into Perfect Chaos. As a result, it became a massive water creature. Floods destroyed the civilization, and Chaos avenged its Chao brethren. In order to prevent Chaos from destroying the world, Tikal linked her consciousness with Chaos' via a prayer, and sealed them both into the Master Emerald. It is probably at this point in history that the Emeralds were first called the "Chaos Emeralds".

Sonic Adventure
Chaos lay dormant in the Master Emerald for thousands of years until Dr. Eggman, having read ancient tablets describing the fall of the Echidnas and Chaos, released it by shattering the Master Emerald. Chaos swore to follow Doctor Eggman as it knew that he could get the Chaos Emeralds back for it. Sonic the Hedgehog eventually encountered Chaos as it was attacking a building and several Police officers, and easily defeated them. Because of its appearance, Sonic initially referred to it as "big drip."

Eventually, Eggman's plans, as always, fell through at the hands of Sonic and his friends. Chaos eventually abandoned Dr. Eggman, destroying his Egg Carrier in the process, and collected the Chaos Emeralds for itself. Once they were in its possession, it absorbed the negative powers of the Emeralds once again and transformed into Perfect Chaos to destroy Station Square in a rage. Having only taken the negative energy, Chaos discarded the remnants of the Chaos Emeralds. This proved to be the means to Perfect Chaos's downfall as Sonic absorbed the positive energy of the Chaos Emeralds and transformed into Super Sonic, eventually defeating it and in the process, its evil mindset was returned to its regular state and changed back to its original form. Tikal then approached Chaos, and together they disappeared in a flash of light. Though its whereabouts are unknown, it is presumed that Chaos returned to watching over the Chao with Tikal.

Sonic Battle
Chaos 0 also appears in Emerl's story in Sonic Battle, where it can be challenged. After the player defeats Chaos, it is unlocked as a playable character in Battle and Challenge modes. In-game, Chaos can barely jump and is incredibly slow but its extendable limbs give it a much wider attack range than it appears to have, and has the highest attack power in the game. It can turn into a puddle of water as usual, and can create fountains, tentacles, and orbs of water.

In the story of Sonic Battle, Knuckles the Echidna claims that Chaos appears when there is an upcoming disaster. 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.

Sonic Generations
Perfect Chaos reappears as the iconic boss of the Dreamcast era in Sonic Generations. Sonic fights Perfect Chaos without his Super State, relying on boosts and Homing Attacks to defeat the creature.

Its appearance differs greatly from Sonic Adventure: Chaos appears to have darker reptilian scaled skin, green horns and teeth, and its body doesn't seem to be made completely out of water as with its previous appearances. Chaos' upper body is solid and the lower body is liquid. It has been explained that this was what Perfect Chaos was originally meant to look like (however, due to technological restraints it was not possible at the time). This design of Chaos is actually seen in Sonic Adventure, on the mural in the Lost World stage.

Sonic Forces
In Sonic Forces, when Dr. Eggman discovered the Phantom Ruby, he tested its power on Angel Island, creating an physical illusion of the village of the Knuckles Clan. A part of this restoration, Chaos emerged from the Master Emerald to fight Knuckles the Echidna and Silver the Hedgehog. However, it disappeared as soon as Knuckles and Silver defeated it.

Chaos, alongside Metal Sonic, Shadow and Zavok, was soon after replicated by Infinite, Eggman's newest servant, using the virtual reality powers of his Phantom Ruby. This replica was instated in the Eggman Army as one of the group's foremost soldiers. The Chaos replica soon after made its debut alongside the other three replicas to assist Infinite in defeating Sonic. Afterward, the Chaos replica helped Eggman conquer over 99% of the world.

Six months later, the Chaos replica attempted to ambush Tails in the City while he was repairing E-123 Omega. Before it could attack Tails though, it was defeated by Classic Sonic when he emerged from a wormhole. Hundreds of replicas of Chaos were later present in the outskirts of the Eggman Empire Fortress where they took part in the final battle between the Eggman Army and the Resistance until all the sources of Eggman's replicas were destroyed for good, ending the replicas' existence.

Sonic Adventure 2
In Sonic Adventure 2, Chaos is a playable character in the multiplayer treasure hunt mode, unlocked in the Dreamcast version by getting A Ranks in all of Rouge the Bat's missions, and available at the start of the GameCube version. In this game, Chaos has the ability to glide and dig in the same way as Knuckles and Rouge. He moves slower than the other characters, but his special attacks hit harder with longer-lasting effects. On the Dreamcast version however, he borrows his special attacks from Rouge. Its brain is not visible in this game.

Sonic Heroes
Chaos did not appear in Sonic Heroes, but was mentioned in two lines, at the ending of Team Rose's story, when Froggy and Chocola are rescued, Metal Sonic says: "Chaos data...has been copied." At the final story, where Dr. Eggman says, "It's hopeless! Metal Sonic has combined your data with the power of Chaos and is super strong!" He copied Chaos' data from Big's frog, Froggy, since he was "possessed" by Chaos's tail, along with Chocola the Chao, since all Chao have some kind of relation with Chaos.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Chaos appears as a Sticker:

Sonic Chronicles: The Dark Brotherhood
Chaos did not appear in Sonic Chronicles: The Dark Brotherhood, but played a significant part in the game's backstory. It was mentioned that because Chaos caused the destruction of the Knuckles Clan, it allowed the Nocturnus Clan to reign supreme in the ancient world, before they were drawn into the Twilight Cage, as it had wiped out the Nocturnus Clan's most prominent opponent. It was also mentioned in one line, when Knuckles realized that Shade was an echidna. Here, he revealed that Perfect Chaos flooded an ancient city and killed several hundred echidnas that resided there.

Sonic and the Black Knight
In Sonic and the Black Knight, the name "Chaos" appears as one of the default names when starting a profile.

Mario & Sonic at the Olympic Winter Games
Chaos appears, as an ice version Perfect Chaos, in Mario & Sonic at the Olympic Winter Games as an obstacle in Dream Figure Skating (Sonic World). He also appears in his normal form as a VIP character that will sometimes send you letters and as a sticker.

Sonic Forces: Speed Battle
In Sonic Forces: Speed Battle, Chaos is a playable character who was added to the game's roster in the version 1.0.1 update. He has the following stats that are applied in gameplay: