Justin is a purple jellyfish that hold his large, yellows eyes lined with stalks ang eight tentacles. Behind his neck are orange, green, and yellow piscine fins, orange, pointed ears, a forked tongue, a mouth lined with fangs, a snake-like neck, and a large blue isopod shell on his head.
When Karai pressed a button on the machine, all of the DNA samples (including a jellyfish, an isopod, an octopus, a cobra and a cat) were dumped into the mutagen tank, creating a large mutant mixed with various species. The creature the meowed, making Michelangelo think he's so cute. However, the creature expands its neck, stretches its stalked eyes, and displaying its piscine fins. The mutant then takes out the Kraang as Karai leaves the Turtles to their fate. When Raphael calls the creature "Octo-punk", Mikey tells him to call him "Octo-Eyeball-Jelly-Bug", but then he finds it difficult and decides to fall it "Justin". Justin slams his tentacles on Mikey and Raph, and zaps Donatello with electricity from his eyes. Leo becomes enraged at the mutant as he bravely fights him. When Justin blasted another blast of electricity, Leo uses his katanas as a conductor and slams it into the machine, setting Justin on fire as he flees, believing he has died.
However, it is revealed Justin somehow survived as he is shown to make a cameo appearance in "Metalhead Rewired" as one of the detained mutants captured by the Kraang in order to control them. After Metalhead released all of the mutants, Justin is seen with Spy-Roach as he crawls out of his cell. One of the Kraang-droids shot him, making him fall several feet to the ground. However, he survived and then heads to the portal to escape.
Due to its appearance being difficult to name it, Mikey simply calls it Justin.
Although Justin is mixed with five specimens, there were other specimens that were included in the Worldwide Genome Project (these include crab, narwhal, giraffe, bull, wasp, praying mantis, mountain lion, snake, lizard, beaver, hippopotamus, crocodile, bat, kangaroo, rooster, caterpillar, and other unknown specimens).