Some TMNT stuff really isn't for little kids.
Mondo Gecko first appears—along with Herman the Hermit Crab—as a recruit for the foundation of a Old Hob's new mutant army, with the goal of increasing mutant numbers and protecting mutantkind from human aggression. Because Mondo was mutated by other mutants and treated humanely like a person from the onset, he has suffered relatively little trauma in his otherwise brief sentient life. Mondo has a warm personality and makes new friends easily, becoming closest to Michelangelo and Seymour Gutz. As part of Mutanimal operations, Mondo uses a skateboard that can also be used as a weapon.
In the Teenage Mutant Ninja Turtles: Mutanimals mini-series, Mondo participates in a Mutanimal offensive against Null Group, which results in the liberation of Null-created mutants Man Ray, Sally Pride and Seymour Gutz and a temporary derailment of Madame Null's mutagen program. At first, Seymour's deep melancholy and suicidal personality is unnerving to Mondo, but Herman encourages Mondo to empathize with and reach out to Seymour; he does so, and they soon become friends.
In Order from Chaos, after Splinter kills the Shredder, takes over the Foot Clan, and all of the Turtles minus Michelangelo join with him, Mondo locates Mikey living on the streets, and extends Hob's invitation for Mikey to join the Mutanimals. Mikey already has many friends among the Mutanimals, but his membership is brief as he remains personally suspicious of Hob and his motives. After his first and only official mission with the Mutanimals, Mikey sneaks out to follow Hob to his warehouse. Mondo notices, tails Mikey, and then returns to tell the others at home what Hob and Mikey are up to. When it is revealed that Hob has not been destroying seized weapons, but has been stockpiling them and selling the surplus back to the human black market using Hun as an intermediary, it severely damages the Mutanimals' trust in Hob. Slash and Mikey immediately quit together; Mondo does not quit, but he is among those clearly bothered by what Hob has been doing.
In Desperate Measures, part 1, Mondo's tail is violently ripped off when Agent Bishop uses a mind-controlled Slash to assault the Mighty Mutanimals Headquarters, whose location had been pinpointed by Bishop's partners Darkwater during Order from Chaos; the tail begins to grow back immediately, however. Less lucky is Seymour, whose exo-suit and sole means of life support and independent locomotion is destroyed in the same attack.
As the Mutanimals lack the resources to replace Seymour's suit, Seymour becomes more or less permanently bedridden with improvised life support, but Mondo spends regular time with him, as seen in Mutagen Maintenance. Then in ...And Out Came the Reptiles, when told that Seymour is deathly ill and will soon die without a replacement suit, Mondo is determined to do all he can to prevent Seymour was dying. With a "shopping list" from Lindsey, Mondo intends to raid an Earth Protection Force facility for the materials and technologies needed to construct a new suit, but Hob, pulling rank as Mondo's dad, forbids him from going on a mission where he can too easily be killed. Mondo defies Hob's authority and storms out, promptly recruiting Michelangelo to help him with his mission. They infiltrate the house of an EPF agent at night, but it becomes clear something is seriously wrong with Mondo's objectivity when the usually happy-go-lucky gecko viciously attacks and tries to torture the agent in front of his young daughter, prompting Mikey to immediately force Mondo to retreat with him. Mondo and Mikey find a way to infiltrate the base, but when they reach a room Mondo realizes is where he was temporarily imprisoned after the EPF attack on the Mutanimals, Mondo is traumatically triggered and becomes angry and impulsively violent, ranting against Hob, Null and the EPF alike, and starts smashing up the room; before Mikey can stop Mondo's reckless behavior, the ruckus has already alerted the EPF and Bishop, who now intend to kill rather than capture the mutant burglars. Mikey and Mondo quickly manage to flee and barricade themselves inside an EPF warehouse, but Mondo's tail, which had finally completely grown back, is again severed in the process. In the warehouse, Mondo manages not only to find everything on Lindsey's list, but an impounded Triceraton landing craft which they used to break out of the compound and get away. Mondo returns home with the materials; he and Hob still argue about what happened, but since Mondo did manage to save Seymour's life, Hob decides not to kick Mondo out. However, Seymour hates Mondo for saving his life, wishing Mondo could have just let him die and not live in misery anymore. Mondo emotionally breaks down, not wanting a life without Seymour, and the two reconcile.
In Battle Lines, Mondo participates in a joint Clan Hamato-Mutanimals mission to Burnow Island where the EPF has invaded with the intent of wiping out the Utroms and Triceratons who live there; Hob intends to use this opportunity to try to wrest Slash from Bishop's mind control once and for all. Though Hob manages to destroy Bishop's mind control goggles and free Slash, it becomes quickly clear that Slash has a nuclear weapon surgically implanted inside him, which Bishop has intended to annihilate all mutant and alien life on the island. Slash has Sally fly the Mutanimals' transport over the open ocean and he skydives above it, detonating safely far away from everyone else. When it becomes all too clear that Slash has been killed, Mondo leans against Herman and starts crying.
Mondo is a green lizard with blue eyebrows and black hair. Though he has only been a mutant for a few years, and his previous age is not specified, Mondo resembles and functions at the social development level of a human teenager. He has no one fixed outfit—his clothes vary over time. Mondo has at least one tattoo—a blue punk circle-A symbol on his right shoulder.
In addition to skill using his skateboard as a weapon in combat, Mondo has the ability to camouflage himself with his surroundings like a chameleon, except instantly. However, this ability requires exposed skin, so he must be naked and unarmed to take full advantage of it.
In Desperate Measures, Mondo is shown to have some information technology hacking skills. But his effectiveness at this skill is limited by his imperfect data-mining judgment, like when he extracted data from Darkwater system only to discover later that what he had obtained was a relatively unuseful list of employees from their human resources server.
In ...And Out Came the Reptiles, Mondo demonstrates a bizarre skill: Rapidly shredding the clothes he is wearing so he can quickly bare the skin he needs to camouflage.