Victor Falco

Dr. Victor Falco was a sleazy scientist who used his knowledge and talents for his own selfish gain. After an accident involving a fire and Mutagen, he became  this series' incarnation of  Rat King. Rat King started out as a human scientist named "Dr. Victor Falco", and debuted in the episode  Monkey Brains. He and his colleague  Dr. Tyler Rockwell were working on an experiment that involved  Mutagen, a neurochemical, and monkey DNA when they were approached by the Kraang. When it was mentioned on the news that Dr. Tyler Rockwell was missing, Dr. Falco alluded that he was oblivious to the situation.

Falco pretended to help April O'Neil and the Turtles and gained their trust. The Turtles later figured out what Dr. Falco was really doing; experimenting with mutagen on his former colleague Dr. Tyler Rockwell, turning the latter into a mutant monkey with psychic powers. Falco gained the same psychic powers temporarily by extracting a neurochemical from the mutant's brain and injecting himself with it. With his new power, Falco was able to defeat the Turtles in combat by reading their thoughts and predicting their moves. Donatello finally defeated him by fighting without thinking, but Falco escaped before the Turtles could ask him what he knew about the Kraang.

In episode 13, I, Monster, Falco was shown living in hiding, and continued to work on his psychic neurochemical in a secret laboratory with only sewer rats to experiment on. One day, the rats chewed through some cables and started a fire in the lab. Falco was injured by the flames, but the neurochemical splattered on him, giving him a psychic connection to rats. Falco used his new ability to control the rats in his lab and they saved him from the fire. Some time later, Falco went into hiding in the sewers, nicknamed himself as The Rat King, and began plotting to use his powers to take over New York City. He launched a full scale assault on the city with his hordes of rats. Eventually Rat King became aware of Splinter through telepathy. Knowing that the Turtles would come to stop him, Rat King used psychological warfare to control Splinter's mind. When Splinter fights the Turtles, they are overconfident because they were actually able to land a blow on him (though they lost soon after), but soon realize that Splinter was going easy on them all those years. They manage to snap him out of it and Splinter beats Rat King with two fingers. Rat King was last seen being carried off by his rats. He may still be alive.