He is a scientist animal that was mutated from mutation that makes him very smart into a humanoid crocodile.
He was recruited by the Utroms, his adoptive family that uses the DNA mutation to make him smart. He began to hate Baxter who works for Shreeder who was the worst enemy of the Utroms to seek revenge against him for betrayal.
He was witnessed that Donatello was mutated into an dangerous reptillian monster that rampages everything. He, Baxter and the Turtles work together to return Donatello back to normal.
In 2012, he was a normal pet crocodile bought by a young human boy. When the boy's parents found out, they flushed the crocodile down the toilet and it got captured and mutated by the Kraang. The Kraang were unable to turn him evil, though. He stole a device that powered the Kraang's portals and escaped, eventually befriending the Turtles thanks to Michelangelo. He is seen helping the Turtles against the Kraang, he has scars on his body and he hates the Kraang, as he flies into a rage nearly everytime someone mentions them.
Leatherhead serves as an anti-heroic protagonist in the most of the animated tv series and comics. In the 1987 series he is a villain and the IDW Comics he is an anti-villain.
Leatherhead was originally recruited by Shredder, but in 2003 and 2012–present, he serves as one of the protagonists in all of the appearences of the shows.
In the 2003 and 2012 versions, he mostly serves as a powerful ally to the turtles. In the 2012 version, as mentioned before, he is a short-fused powerhouse at the beginning of the series. However, as Mikey befriends him, he takes on more of a 'gentle giant' role, letting himself be angered less easier and usually hugging his friend when they meet again. Mikey also gives him his name in the 2012 version. After his debut episode, his next appearance has him help the Turtles in locating his power cell after they lost it to the Kraang. Near the end of the episode, he suddenly joins them in a big battle and stops the monster Traag. Realising something bigger is coming through the portal, he says goodbye to the Turtles and enters the portal with Traag in tow, warping them both to Dimension X and stopping the other threat. He also makes a cameo appearance as a "dragon" in the Mazes and Mutants episode. The episode "Into Dimension X" reveals that, due to the time flow difference in Dimension X, Leatherhead has aged rapidly, his scales graying in the process. He steals a communication orb and contacts the Turtles to stop the Kraang because they perfected the mutagen. With help from Mikey, he is returned to the normal dimension. In the second part of the Season 2 finale, he saves Splinter from getting killed by Shredder, but the latter defeats him and sends him to drown in a small body of water. Him being a crocodile, he naturally survives. He later teams up with Slash, Dr. Rockwell and Pigeon Pete (who is later replaced by the equally incompetent Mondo Gecko) to form the Mighty Mutanimals with the help of the photographer Jack Kurtzman