Victor Fries, also known as Mister Freeze, is an enemy of Batman's. He is normally restricted to a suit that keeps his body temperature low, and uses ice and cold as weapons. At some point prior to the events of Batman vs. Teenage Mutant Ninja Turtles, he was captured and imprisoned in Arkham Asylum.
He was one of the criminals seen by Shredder and Ra's al Ghul on their excursion through the facility, and he requested that he be left out of whatever they were planning. He was later freed by the Joker, and sprayed with mutagen that turned him into a mutant polar bear who no longer needed his temperature-controlling suit.
When searching for hostages, Michelangelo, Raphael and Robin encountered Fries. He was now armed with an ice gun that he fired at the Turtles and boy, and proved to be immune to ordinary physical attacks due to his mutated physiology. Eventually Robin immobilized him with some heat-producing explosives, but this proved to be a temporary measure.
His foes then eluded him for a time, but Fries caught up to them as Batman was just about to fight Harley Quinn. Despite another attack by Robin and Michelangelo, Fries focused mostly on attacking Batman. He eventually immobilized the superhero with a thick layer of ice, allowing the Joker to inject him with a mixture of mutagen and Joker Venom. This mixture made Batman not only a bat mutant, but violently insane.
Batman then attacked Fries, shrugging off his ice attack and smashing him through multiple floors under the asylum. The results of their conflict are ambiguous; he may have been killed by the crazed Batman, or he may have been demutated by the police once it was confirmed that the retromutagen was non-lethal.