Some TMNT stuff was always meant for grownups.
Galahad (formerly "Kid Galahad") is a superhero in the "Big Bang Comics" universe. He was formerly the sidekick to the superhero Knight Watchman, but struck out on his own after Knight Watchman retired.
Galahad's real name is Jerry Randall, and in his citizen identity, he is a lawyer working under the District Attorney of Midway City. The Knight Watchman, Reid Randall, was his uncle.
Big Bang Comics #10
In the story Galahad, Jerry tracks down the assassin Headhunter, the murderer of Reid, to New York City. He interrupts a man and woman in mid-coitus and begins attacking the man, who he believes is Headhunter, and tells the woman to lock herself safely in the bathroom. During the fight, the woman re-emerges, revealing herself to be Headhunter, and the man who Galahad had mistaken for the assassin is actually the Japanese crime boss Oroku Saki. Saki steps forward in his full Shredder armor and orders his Foot Soldiers to kill the pair.
Galahad and Headhunter call a truce and begin battling the Foot Clan. Galahad fights his way to the Shredder and explains that he’s only after Headhunter. Shredder refuses to give her up, saying they have unfinished business. Galahad then takes three arrows to the back and a bo staff to the head. As Galahad falls face first to the ground, Headhunter berates him for not taking the Shredder down when he had the chance and angrily calls their truce off. As Headhunter escapes, Shredder orders two of his Foot Soldiers to finish off Galahad while the rest go after Headhunter and bring her back alive. The Foot Soldiers decide to do a half-assed job and just dump Galahad’s body into the sewers.
Downstream, a very, very young Michelangelo and Leonardo are fishing. Mikey snags something that turns out to be Galahad. Raphael then shows up. Unimpressed with yet another dead body in the sewers, the three Turtles fish him out and argue over who he might’ve been. As Leo attempts to remove Galahad’s mask, the superhero comes to life and grabs Leo’s hand. Raph is about to beat him with his stick when Splinter intervenes. Splinter and the Turtles then aid the injured and confused Galahad back to their lair.
Reviving Galahad with some herbal tea, Galahad reveals that his body armor stopped all but one of the arrows. Leo identifies them of ninja design and then all four Turtles begin fighting over it. Galahad explains to Splinter that he had a run-in with the Foot but then finally breaks down and has to inquire just what the Hell is up with all the talking animals. Splinter tells his origin to Galahad: Hamato Yoshi, the Foot Clan, Oroku Saki, the ooze, you know the drill. Galahad is surprised to hear the name Hamato Yoshi, as his mentor had trained under him as well. Splinter than leads him to just below a shoe store that’s secretly operated by the Foot and bids him good luck on his confrontation with the Shredder. Galahad thanks Splinter for his help and leaves. Splinter sighs that Galahad’s code of honor will not allow him to kill the Shredder and resolves to teach his children the more deadly arts for their inevitable confrontation with their foe. As they go back to the lair, the Turtles all agree that Galahad’s mask was just awesome and decide to make some for themselves.
Galahad cautiously enters the shoe store and finds seven Foot Soldiers dead; slashed to death presumably by Headhunter. Galahad then finds the Shredder, sitting behind a desk. Shredder asks if "the White Knight" has come to rescue his damsel in distress, but Galahad insists he’s here to arrest her. Shredder is reluctant to give her up, as she’s a threat to his life and he doesn’t believe any prison can hold her. Galahad insists, leaving Shredder no choice…but to hand her over? It seems that he owes Galahad a debt of honor for (accidentally) saving his life when he was in bed with Headhunter. At any rate, he’s done with Headhunter, anyway. After an interrogation, it seems that seven of his Foot soldiers, displeased with Shredder’s leadership, hired Headhunter to deal with him. Shredder, in turn, dealt with them. Shredder then tells Galahad to go collect Headhunter and then never cross his path again, lest he be forced to kill him.
Galahad finds Headhunter in an interrogation room, badly beaten and with a sai sticking through her hand. Galahad arrests her and is about to bring her to a prison hospital when she offers "a deal". Galahad refuses anything she has to offer, so Headhunter responds by knocking him out. Giving him a kiss, she promises to come back for him some day after she’s healed from her interrogation.
A few days later, Galahad swears over Terry’s grave that next time Headhunter shows her face anywhere in the world, he’ll take her down.
Jerry Randall reappears in issue 9 of the Image run, under the guise of Knight Watchman.
Following a lead on the whereabouts of Splinter, Leonardo flies the Triceraton Aircar into Midway City. While reading the city's local newspaper, he's attacked by a mutant bat that was sent by Splinter to find and attack Leonardo. Leonardo fights the bat, taking the battle down to the streets. As he is about to stake the bat to death, he is stopped by superhero Knight Watchman. Watchman tells Leo that since the mutation is spread by rabies, a rabies vaccine also reverses the mutation, as he injects it into this mutant bat. Watchman then reveals to Leo that they had met before - and Leo realizes that Knight Watchman was who he knew as Galahad.
Knight Watchman tells Leo that once the "lab boys" determined that mutagen was involved, he tried to reach out to Splinter and the Turtles for help, but he was unable to get in contact. Leo explains how Splinter is involved in all this and how he was somehow mutated further into a mutant bat by Dr. Wu's synthetic mutagen.
Leonardo unfortunately comes to his aircar and finds that it's been stripped for parts. Luckily, Knight Watchman had already come across the goons doing it and tied them up, although Leo doesn't know how to put the pieces back together.
Back at Knight Watchman's civilian residence, he and Leo discuss his identities as Galahad and Knight Watchman. Jerry Randall operates as Galahad during the day and Knight Watchman at night, taking the latter mantle from his retired uncle Reid, as sort of a good cop/bad cop routine.
Leo tries to contact Splinter on the astral plane, but as he meditates, suddenly a mutant bat and a bunch of regular bats crash through Randall's window. Randall fights off the bats and just as he is about to inject the mutant with the antidote, the original mutant bat barges in. This mutant bat flies out Randall's window and reverts back into a human, but is too far for Randall to reach. Fortunately, Leo breaks meditation and rescues the man with his kaginawa. The other mutant bat then escapes, but Knight Watchman had planted a tracking bug on him in hopes that he would lead them to Splinter.
They follow the trail to a closed-down club called "The Mine Shaft". Inside, they find scores of mutant bats. Knight Watchman gasses the place and injects them with the antitoxin, while Leo handles Splinter. Splinter tries to fly away, but Leo hitches a ride on his back. Leo manages to inject Splinter with the vaccine, but he is tossed into a tree. Knight Watchman catches up to him and Leo laments their failure, as Splinter flies out of town, seemingly unphased. Knight Watchman says that they saved the city and he's proud of Leo, as the turtle drops out of the tree onto his head.