As a human, Kirby was bald yet had some orange hair and a beard. He had blue eyes and wore a brown jacket with a grey undershirt, along with tan pants and brown shoes.
In his mutated form, he has a large, mis-shapen head with a large green vein running down it. His red beard is still present. His ears look like bat wings. He has green eyes, a flattened bat nose and razor sharp teeth. He is covered with orange fur on his lower body down to his legs. His right arm is small while his left appears as a normal size. He has a long purple and green vein running down his body. Finally, he has long purple wings with an arm attached to the top of each. Each hand has claws. He also has a short tail.
In this version April father is still alive and named Kirby O'Neil in the 2012 animated series, but is no antique dealer but instead a scientist of psychology and a widower, which the Kraang kidnapped, who after the death of his wife had to raise his daughter April alone.
When the turtles are out in the city for the first time in Rise of the Turtles, Part 1, they witness Kirby and April being abducted by the extradimensional Kraang. Because the aliens wanted to use April's unusual psychic abilities to colonize the earth. The turtles attempted to save them, but due to their inability to work as a team, the Kraang escape with them. Later, when the turtles find Snake and question him about the mutagen and why the O'Neil's were taken, they believe that Kirby is being used to help the Kraang. Once they break into the Kraang's lair, they manage to save April, but were unable to get Kirby.
In The Gauntlet, Kirby sends Pete to deliver a message to April warning her to get out of the city. Instead, April and the Turtles get Pete to take them to where the Kraang are holding Kirby in order to rescue him. Kirby recognizes the Turtles and tells them what little he knows about the Kraang and their mutagen. The rescue mission is interrupted when Donatello discovers the Kraang have set a time bomb to disperse mutagen over the city. The Turtles free Kirby from his cell, but the Kraang soon corner them. Kirby takes one of the Kraang's guns and attacks the droids, telling the Turtles to save April and the city. He covers their escape, and is last seen being disarmed and re-captured by the Kraang.
In TCRI, April reveals what kind of scientist her father is; a psychologist. This means Kirby does not have the type of skills or knowledge the Kraang would find useful. Just when Raphael wonders why the Kraang would kidnap him in the first place, Donatello reveals that Kirby wasn't their real target. Instead, it was April the Kraang wanted, and they're still looking for her. It's not known why April is the Kraang's primary target or if Kirby knows why the Kraang are after his daughter.
In the episode Operation: Break Out, he sends April a coded message, which Donatello decodes. He mentions that he was moved to a different location and includes the coordinates. Donatello goes to the lab and manages to break him out, but he also lets loose a giant salamander monster. Kirby and the turtles manage to avoid the Kraang and defeat the Newtralizer and successfully escape. Once they are back at the sewer he is reunited with April. At the end of the episode, he and April hug each other and he evilly looks up, indicating something is wrong. The exemption was part of a perfidious plan of the Shredder, who had now joined with the Kraang in an alliance against the Turtles.
In "Showdown, Part 1", the Kraang control Kirby into handing April over to Karai and the Foot Soldiers. In "Showdown, Part 2", the Turtles discover the mind-control device the Kraang gave Kirby on the back of Kirby's neck and free him and give them important information about the plans of Kraang Prime, the top leader of the Kraang, the information gave is what led to the defeat of the first invasion.
However, the long imprisonment had caused major trauma to O'Neil, and his desire to protect his daughter from harm, thus has become an obsession. In "The Mutation Situation," When the Turtles finally learned of a new plan of Kraang and asked April for assistance they had to trick her father to let them take her out of the house her father. Kirby has been having recurring nightmares about the Kraang and helps April to track the stealth ship. But on this occasion it happened that the Turtles in their imprudence caused a whole load of mutagen, which the Shredder wanted the Kraang to deliver, to rain down on the city. When a mutagen canister falls off the ship and towards April. Kirby protects April from it and falls off of the roof of the building. As a result of Kirby falling past a bunch of bats, Kirby mutates into a grotesque mutant bat with a large head. Michelangelo tries to call him "Wingnut" as a nickname, but is scolded for his insensitivity. Although his human intellect has been largely suppressed by the effects of the mutagen, O'Neil's protectiveness towards his daughter remained, and he therefore sought to continue to keep his daughter from harm. During the fight, the Turtles direct him into the warehouse where he is caged. When April catches up, Michelangelo unknowingly mentioned how the falling mutagen canisters was an accident on their part during their fight with the Kraang. Kirby breaks free and flies away to an unknown destination in the nocturnal New York. The mutation of Kirby had put a strain in April's friendship with the Turtles. She parted ways with her mutant friends.
Kirby seems to be smart and very protective of his daughter whenever her safety is threatened. He also acts very selflessly as he sacrificed his freedom to save the turtles and his daughter, so that they could save New York City from the Kraang's mutagen bomb. After Operation: Break Out it is revealed that he was brainwashed by the Kraang. Even after his mutation, he is shown to still care about April, having brought her food.
- He is most likely named after legendary comic book artist, Jack Kirby, whose name was also given to the character Kirby from the Mirage comics and 2003 TV series. Though his appearance (and his profession as a scientist) faintly resembles August O'Neil's from the 2003 TV series.
- He's the first adult human to be in the lair.
- In the episode The Mutation Situation Mikey calls him Wingnut, as a reference to that character.
- The Kirby Bat action figure was released in 2014 by Playmates.