The Sliver is a supervillain Rat from an alternate reality. He is Splinter's evil alternate dimension counterpart and the grueling, destructive former sensei of the Super Turtles. His blind loyalty to his Master Shreddarius has put him at odds with his students, and he has become one of their major, though reluctant foes.
In another universe, where the Turtles are superheroes instead of ninjas, their master, referred to as the Sliver, was initially their foster father but was eventually corrupted by evil. When Michelangelo was stranded in this parallel dimension by the fused Ultimate Ninja and Drako, Sliver was planning to wipe out humanity with the Penultimate Nullificator. However, because the device destroys all life on the planet, the Sliver modified it so that his tower and everything inside would be spared. He also planned on turning the Super Turtles to the side of evil once the Nullificator was set off, catching them in a Utromidium cage that proved impervious to their powers.
However, Michelangelo, who had accompanied the Super Turtles, came up with a plan to set them free. By having Blobboid pose as him, he tricked the Sliver into thinking that in Mikey's home universe, he and his brothers ruled the Earth and that they were loyal to their master. When the Sliver released 'Mikey', Blobboid revealed himself and snatched away his control cane, releasing the other Turtles from the cage. Unfortunately, the Sliver had the same powers the Super Turtles had and had mastered them to a degree which made him superior to his students.
With the Super Turtles quickly incapacitated, Mikey was left alone to face the Sliver. Using his ninja skills, he trapped the Sliver in his own cage, but the Nullificator was still set to go off. Luckily, Mikey figured that they could reverse its function to go off inside the tower. Shellectro managed to reprogram the device, and the Super Turtles escaped before the Nullificator destroyed the tower with the Sliver inside it then went to another nowhere, while Mikey was pulled away from this dimension by the efforts of his brother Leonardo to reunite his family.
His armor is very similar to the Shredder's.
In Teenage Mutant Ninja Turtles 3: Mutant Nightmare The Sliver makes a guest appearance as a boss in a later shooting, first-person perspective portion of the game. Similar to the effect of the episode Reality Check, the Turtles are blasted into other dimensions by Ultimate Drako. However, in the game the Teens fight in each universe and face off against The Sliver together.