Metalhead (Teenage Mutant Ninja Turtles)

Metalhead is the fifth Ninja Turtle of the "Teenage Mutant Ninja Turtles" franchise. His colored mask is yellow. 
"Metal Head" originally began as a superhero with the power to control his fluid metal hair like tentacles, as well as a member of the Justice Force from the Mirage comics (which later serves a basis for the 2003 incarnation as well.)
In the 1987 cartoon series, a separate character named "Metalhead," a robotic turtle, was created by Krang, which would later serve as a basis for the 2012 character of the same name but with a different backstory.
Comics.
Mirage.
In the Mirage Studios' "TMNT" comic "Dome Doom", he is currently retired from super-hero business and lives in Northampton, Massachusetts. In volume 4 issue #6 Shadow's boyfriend, Jay, decides to investigate the farmhouse where she lives in order to get a look at her mysterious grandfather. The boy takes a rifle with him to utilize its scope to get a closer look, and he's discovered by a very unhappy Metal Head.
IDW.
In the IDW comic, Metalhead is constructed by Harold Lillja as a robot able to fight controlled remotely. It is later used to store Donatello's consciousness after he is critically injured during a fight with Bebop and Rocksteady, and destroyed when Donatello transferred back to his own body.
Donatello later rebuilt Metalhead, and was shocked to discover a copy of his own consciousness existing in the robot body. This consciousness, briefly called "Metal-Don," became mentally unstable because of the pain of his PTSD and his inability to process his emotions. After attacking Donatello, Metalhead transferred himself into a new robot body, erased his emotions, and escaped the Turtles. He later decided to eliminate the Turtles in exchange for access to Area 51's computers, needing an outlet for his incredible mental abilities.
Cartoons.
1987 cartoon.
In the 1987 series, Metalhead first appeared in the season 3 episode "The Making of Metalhead" where it was imbued with the thoughts and memories of the Ninja Turtles. This gave Metalhead an identity disorder until Donatello reprograms him to work for the Ninja Turtles. In this appearance, Metalhead replicated the voices of the Ninja Turtles, Shredder, and April O'Neil, meaning that Metalhead was voiced by Cam Clarke, Barry Gordon, Rob Paulsen, Townsend Coleman, Dorian Harewood (who was filling in for James Avery at the time,) and Renae Jacobs. He was only seen again when Donatello tries to get him to vacuum their TV room in the season 4 episode "Big Bug Blunder."
2003 cartoon.
In the 2003 series, he was a member of the original Justice Force and remains a member of the current Justice Force, making him the only member to serve in both incarnations of the team. Neither incarnation seems to have aged in the intervening decades; in the case of the first incarnation, it was stated that this was due to his artificial nature. Metal Head was also part of the final attack on the Tengu Shredder. Metal Head appears in the "Fast Forward" season episode "The Journal". Metal Head also appears in . He shows up in the episodes "Super Power Struggle" and "Wedding Bells and Bytes."
2012 cartoon.
In the 2012 "Teenage Mutant Ninja Turtles" series, Metalhead was created by Donatello from a salvaged Kraang droid, creating Metalhead to be their robotic brother. Donatello could control Metalhead via a NES-like controller while observing the situation through a TV monitor since there are cameras in Metalhead's eyes. Also, Donatello can speak through Metalhead. While Metalhead proved to be nearly unstoppable in combat situations, he was far too clunky and heavy to properly sneak around. Thus, this left Metalhead to be forced to sit out a mission against the Kraang. When his brothers became overwhelmed, Donatello sent Metalhead in. He easily defeated the Kraang attacking them, knocking off the control antenna in the process. One Kraang realized that Metalhead was made from their technology and detached from its robot body where it opted to control Metalhead. With the signal lost without the control antenna, Donatello could not defend Metalhead from the Kraang's control. The Kraang-controlled Metalhead attacked the Ninja Turtles, nearly killing them until Donatello showed up in person. Donatello managed to defeat Metalhead by crashing a support pillar on it. The Kraang controlling it ran off. In "Showdown" Pt. 1, Metalhead was rebuilt by Donatello where it aided the Ninja Turtles in the battle against the Kraang. Apparently now programmed with some degree of independence and operating without any sign of a direct controller, Metalhead drove the Shellraiser into the TCRI building lobby and subsequently attacked the Kraang guards while the Ninja Turtles used their new gliders to infiltrate the building from the roof.
Metalhead is reconfigured by Donatello in "Metalhead Rewired", in which he exhibits a personality similar to R2-D2 - being a sentient, small and extremely helpful droid in the service of the heroes. After a battle with the Kraang goes awry, Leonardo, Michelangelo and Raphael vote to terminate Metalhead, who quickly escapes before Donatello can do so. Metalhead is able to find a portal to a Kraang prison where many mutants from the series have been incarcerated. Realizing Metalhead's benevolence, the Turtles fight alongside him to both free the imprisoned mutants and to return to New York. However, Metalhead is forced to remain behind to keep an escape portal open, and he self-destructs in order to destroy the portal device and the Kraang swarming him. The Turtles and the various mutants are able to escape, and Donatello finds Metalhead's lifeless head. Grieving, Donatello ponders over one day being able to rebuild Metalhead.
A second version of Metalhead is inhabited by Donatello's consciousness in the alternate-universe three-part special "Raphael: Mutant Apocalypse."
Video games.
Metalhead appears in "" as the boss of the second level Alleycat Blues. He has extending punch and kick attacks, as well as a machine gun in his chest.
This version was slightly redesigned for the remake, "Turtles in Time Re-Shelled"; this version of Metalhead was voiced by Wayne Grayson. The instruction manual of the Super NES version of "Turtles in Time" erroneously refers to Metalhead as Mechaturtle.
Toys.
Metalhead appeared as part of both the 1989 and 2012 toy line.