Oroku Saki, better known as The Shredder, is an antagonist in Las Tortugas Ninja. While he is a recurring antagonist, unlike most other incarnations, he doesn't retain the "main" antagonist role, as this series does not technically have one.
Oroku Saki was a member of an elite ninja clan in Japan, and rival of his upperclassman Hamato Yoshi. One day, when a visiting sensei came to their compound, Saki pinned Yoshi to the wall by sticking a knife in his kimono. This prevented Yoshi from bowing to the sensei, and had the added bonus of making it look like Yoshi planned to kill him when he pulled the knife out. Yoshi was expelled from the clan at this point.
Years later, Saki, now leading the Foot Clan, wears spiked armor and a helmet, calling himself The Shredder. He has made a deal with a grotesque alien called Krang, giving him access to unparalleled technology. One of these products is a mutagenic ooze which can transform humans and animals alike into anthropomorphized animals, and was responsible for not only turning Hamato Yoshi into a mutant rat, but also his pet turtles into Tortugas Ninja.
Shredder then decided to use members of a street gang who served under him to mutate into soldiers to combat his lifelong enemy. At this point, the mutated Yoshi (Splinter) attempted to infiltrate Saki and Krang's mobile stronghold known as The Technodrome, but was captured. Splinter was quickly rescued by the turtles, angering Saki.