Leatherhead

Instead of a villain as was his previous animated incarnation, Leatherhead was an ally to the turtles with a scientific mind like Donatello. He is voiced by F.B. Owens in seasons 2 and 3 and by Gary Lewis in seasons 4 and 7. Leatherhead first made a shadowed cameo in Secret Origins, Part 3 escaping from the TCRI building during the raid. He was first seen in the episode What a Croc, where he was helping Baxter Stockman create a new body, while working on a transmat device so he could rejoin the Utroms and had built the Turtlebot. Leatherhead eventually met and befriended Michelangelo who found him living in the Turtles' old lair. Unfortunately for Stockman, the doctor accidentally allowed Leatherhead to know he once worked for Shredder; since the Utroms had adopted and told him about the Shredder, Leatherhead immediately got angry. Eventually Stockman bombed the location and despite Michelangelo's pleas for him to follow them Leatherhead sacrificed himself to save the Turtles. Later in season 3 it was revealed that Leatherhead had in fact survived the ordeal, but had been taken captive by Agent Bishop. The Turtles soon helped Leatherhead escape and for a time lived in the Turtles' lair. However, Leatherhead's animal rages, already dangerous, were made more so by Bishop's experimentation. While in blind animal rage, Leatherhead struck Michelangelo, and woke up to believe that he had killed his friend. Leaving the Turtles, a depressed Leatherhead found himself hunted by an extreme game hunter, Mr. Marlin, through the sewers. Luckily, he was saved by his friends who refused to give up on him, and was delighted to discover Michelangelo was alive and well. In the end, he chose to live away from his friends due to his affliction, living in a nearby chamber (an abandoned subway) so he would always be nearby, but still able to ensure their safety. He reappeared in a number of subsequent episodes as an ally to the Turtles primarily in their battles with the Foot and Agent Bishop, including the raid on the Shredder's building facing Hun, during the battle the two fell into the silo. Both survived, and Leatherhead returned to his lair but lived by himself to avoid accidentally hurting the Turtles due to his anger issues that furthered increased from Bishop's experiments on him. He is later revealed to have helped Donatello create the Monster Hunter gear used by the Turtles against Bishop's mutant creatures.

During the fourth season, Leatherhead played an important role in working together with Don in containing the mutation of local creatures from the mutagens that were created by Baxter Stockman previously during his faked alien invasion. However, when Donatello is accidentally exposed to the mutagen, Leatherhead works with the other turtles in capturing the mutated Don. Leatherhead later discovered that the combination of the mutation from Bishop's outbreak was mixing with the original Utrom mutagen in an unstable way, it would kill Don unless if it was cured.

Leatherhead worked with the Turtles and Splinter in breaking into Area 51 where Bishop was. While Bishop sent the three turtles to steal the Heart of Tengu from Karai, he was forced to fight the mutated Donatello. Reminded of the trauma he had suffered, he was tempted to extract revenge on Bishop but managed to control his anger with the help of his friends. Despite his personal grudge against Bishop, Leatherhead was able to use the details that Bishop and Stockman had on the new mutagen, and using Bishop's resources, he is able to cure Don, and the other animals mutated from the outbreak.

Biography
In the original Mirage comics, Leatherhead presumably started life as an exotic pet which ended up in the sewers after being flushed down a toilet. He was found by two Utrom scientists and taken back to their lab at the TCRI facilty for study. While there he was accidentally exposed to a mutagen and grew larger and more intelligent. The aliens decided to educate the creature and allowed him to live with them.

Despite acceptance by the Utroms, Leatherhead still had a strong connection to the sewers and would go exploring the underground tunnels for days before returning to his alien friends. One day however he returned to find the TCRI facilty destroyed. Assuming humans were responsible and hoping that the Utroms had teleported back to their homeworld, he gathered what little technology was left and retreated into the sewers, working on a device to contact the Utroms.

Encountering the Teenage Mutant Ninja Turtles in the sewers while they were being attacked by a big game hunter called Marlin who was ironically searching for a giant alligator that had been sighted in the sewers. The Turtles and Leatherhead realized their connection and after defeating Marlin the Turtles allowed Leatherhead to live in their original lair which they had since abandoned so he could work on his device in seclusion.

Years later the lair is found by a group of Foot Clan members. Leatherhead who now has an eye patch attacks the intruders by a lone Foot ninja convinces Leatherhead to accept them as allies. The mutant reveals that he's been working on a transmat to teleport him back to the Utrom homeworld but his over-sized hands make it difficult to do so but now that he had friends with smaller hands he can finally achieve his dream. The Turtles arrive at their old lair and a fight breaks out with the Foot only to be diffused by Leatherhead and both groups grudgingly work together to finish the transmat. Once the device is complete the gator steps in but when activated he is not teleported away, but rather burned by energy. In a violent rage Leatherhead starts to lash out at Foot and Turtle alike swearing vengeance on both. Years later in Tales of the TMNT Vol 2 Leatherhead came across Raphael in the sewers and in a primal rage attacked him. Coming to his senses before he could do any permanent damage he explained that he was suffering from uncontrollable fits of rage where pure instinct would take over. More mysterious was the fact that his eye had healed, he'd grown much larger and would black out for days with no memory of what transpired. He showed Raphael a completed transmat but explained he had no recollection of building it. After the arrival of the other Turtles, the machine activated and a group of Utroms with ray guns captured Leatherhead teleporting him away, and they left behind explosives which destroyed the Transmat behind them.

The story was followed up issues later revealing that the Utroms that had captured Leatherhead were part of a secret religious sect called the "Illuminated" who believed it was their god given duty to cleanse the universe of all violent species so that peaceful ones can live without fear or hindrance. They had been secretly experimenting on Leatherhead to make him the perfect weapon so now they could make an army of his clones to attack other worlds.

Thankfuly Leatherhead was rescued by a group of Utroms led by an Utrom called Ribky and accompanied by the Turtles. After fighting their way out of the Illumitated's, Ribky stayed behind and sacrificed himself to destroy the base. The other Utroms explained that Ribky was in charge of waste disposal at the TCRI facilty and so was responsible for the accidents that mutated Leatherhead and the Turtles. Feeling responsible for his creations he had to make sure they were all okay before he died. In the end Leatherhead requested to be sent home to Earth.