Some TMNT stuff really isn't for little kids.
Victor Fries was a scientist who was researching a cure for his wife's terminal disease. An accident with the cryogenics he used to keep her in stasis permanently changed him, giving him tolerance of cold and powers rooted in ice. Now he uses his talents to commit crimes to fund his research under the alias Mr. Freeze, often coming into conflict with Batman.
In the first Batman/Teenage Mutant Ninja Turtles mini-series, Freeze was one of several super-villain inmates incarcerated in Gotham's infamous Arkham Asylum who was exposed to mutagen as part of a plot by the Shredder and Ra's al Ghul to take control of the city with a mutant army. Freeze was mutated into a polar bear, increasing his strength and allowing him to survive in normal temperatures without relying on his cryogenic suit.
He did not apparently resent his transformation, being more focused on punishing Batman for their past encounters. He successfully froze Batman in place, only to end up fighting Michelangelo, whose mocking tone irritated him. Michelangelo successfully redirected Freeze's freeze ray, and knocked him senseless. It was later confirmed that he would revert back to normal in a week's time.
Freeze reappeared briefly in Batman/Teenage Mutant Ninja Turtles II, where it was revealed that he had been attempting to build a portal in order to reach other dimensions. Donatello and Damian Wayne successfully thwarted his plans and took his equipment for their own ends.