World Entrance Level 2

Metal Sonic Kai is the final boss of Knuckles' Chaotix. It is a huge, red version of Metal Sonic, with a slightly elongated, cat-like face.

History
After losing to the player, Metal Sonic comes to Dr. Robotnik. With his Special Ring, the Doctor motions Metal Sonic and escapes. Suddenly, through an apparent transformation that occurred off-screen due to the power of the Special Ring (although this is unconfirmed), Metal Sonic seems to reappear as a huge, red, cat-like mechanical being, and the character (whichever the player chooses) and his partner fall into a square room with ten Rings in hand to fight Metal Sonic Kai.

Regardless, it must be defeated in phases because of its large size. During the first section, hit the body eight times to destroy the left hand and Metal will go into the background and attempt to hit the player with his right hand. After another eight hits, his right hand will be destroyed and the robot will go to the left of the arena and will fire a huge laser, which can be dodged by ducking. After eight hits, the robot will be defeated and if the player has all six Chaos Rings, the good ending will play with the Rings disappearing and Sonic and Tails appearing in the Tornado behind the Chaotix Crew (Sonic and Tails' only appearance in the game). If the player fails to collect the Chaos Rings, Metal Sonic Kai will appear overlooking a city burning in flames (possibly Newtrogic High Zone).

This fight can be very easy if playing as Charmy and Heavy, as Charmy is invincible while flying and Heavy is invincible in general.

Sonic the Comic
Metal Sonic Kai was thoroughly re-imagined in the series Sonic the Comic, and replaced by Emperor Metallix. Both were large, red versions of Metal Sonic (called Metallix in this continuity), but Emperor Metallix was a separate unit. He was the leader of the Brotherhood of Metallix.

Emperor Metallix first appeared in the story "Total Chaotix" from Sonic the Comic #53 to #58, a loose adaptation of Knuckles' Chaotix.

Archie Comics

 * Main Article: Titan Metal Sonic

This boss machine appears as a conclusive transformation of Metal Sonic, in the Knuckles' Chaotix special from Archie Comics. He is defeated by a similarly transformed Knuckles, who used a Power Gem to defeat him since the Chaos Rings were completely absent in the comic.

It made a second appearance was under the name of Titan Metal Sonic. It was an upgrade of Metal Sonic v3.0. It was launched by the Death Egg Mark 2 to destroy New Mobotropolis. It was defeated by Ixis Naugus, who use his Ixis Magicks to turn the robot into a Crystal Golem. Naugus then used the Golem to block a blast from the Death Egg's annihilator beam, destroying the robot in the process (StH #231).

Titan Metal Sonic was seen being rebuilt during Snively's escape. This model was later completed and fused with the Krudzu to combat Team Fighters. It was defeated when the team all attacked it at once, sending it into the ocean.

Trivia

 * Metal Sonic Kai is apparently Metal Sonic's first transformed incarnation. Fleetway Editions imagines it as a separate robot character, whereas Archie Comics considers it a transformation - the original game itself is unclear.
 * In Sonic Universe 37, Titan Metal Sonic's head can be seen by where Snively and the EggSwats are.
 * By hacking into Knuckles' Chaotix, it is possible to find an unused sprite of Metal Sonic Kai's head with several tubes hanging from the "neck". This could have been used as part of his escape sequence should the player fail to collect all the Chaos Rings.