Mondo Gecko is a classic character in the 2012 series of TMNT. He's a mutated gecko who is a good skateboarder (but not a good fighter).
Jason was skateboarding on his own self-made halfpipe when a canister of Mutagen came falling from the Kraang ship, and one of them struck him on the head. Having placed pet leopard gecko Lars on his shoulder, he mutated into a 4-foot-tall humanoid leopard gecko and was rejected out onto the streets by his parents. Months pass and Jason spent them skateboarding alone in the shadows until one day, after the Kraang invasion, he was taken in as an errand boy by Fishface and wows Michelangelo with his awesome skateboarding tricks. Jason, nicknamed "Mondo Gecko" from then on, was taught that it’s every mutant for himself, but after seeing Mikey and Casey Jones battle Fishface, who then exposes his true nature by slapping Mondo away, he learns that it is always better to have a friend's back, and so starts the unbreakable bond between turtles and geckos.