Some TMNT stuff was always meant for grownups.
Cha Ocho first encounter with Leonardo occurred when he was nine years old. His mother and him were returning home at night, and, despite his mother's warning, he lead her into an alley. They were approached by potential muggers until Leonardo appeared out of the shadows. He quickly dispatched the muggers.
Amazed at Leonardo's skill he yells, "I... I want to be just like you!" Leonardo replied, entering the shadows, "No, you don't."
Cha Ocho happily marries his sweetheart Kate, and he is at the top of his game with a budding career as a cop in Manhattan.
All of this is shattered when Cha attempts to help some Transit cops catch a mugger. In the chaos, the mugger turns around and fires a single shot. This shot hits Cha's wife who was waiting with him before he ran after the mugger. The impact knocks her into an oncoming subway train, and Cha vows for vengeance.
After a year, the case of his wife's murder runs cold. Upset at the prospects, Cha decides to take justice into his own hands, and he quits the police force.
In order to strengthen his skills, Cha joins the Foot Clan. After he has broken into the police station to get his wife's case, he runs into Leonardo, instantly recognizing him from his childhood.
He convinces Leonardo of his honest motives in solving his wife's murder, and Leonardo agrees to help him. Leonardo even brings him to the lair that all of the turtles and Master Splinter share together in order to examine further clues in the case and come to the conclusion that Lawrence McKinney is the likeliest suspect, as he “fell off the face of the Earth” shortly after the time of the murder and was never able to be found for questioning.
Cha Ocho and Leonardo track down Lawrence McKinney to St. Thomas Church where he and his wife were married. Cha finds Lawrence homeless and sleeping on the steps leading up to the church. Before, Cha is able to commit the killing stroke for the first time, Leonardo stops him. However, Cha quickly escapes and charges after the fleeing Lawrence. After catching up to him, he cuts off his hand and then kills his wife's murderer.
Leonardo accosts Cha Ocho about there being no honor in vengeance and killing an unarmed, barely capable man. Leonardo decides to teach him a lesson in honor and fights Cha with a pocketknife. Leonardo does the seemingly impossible by shattering Cha's sword with the pocketknife, and he quickly slices Cha across the face leaving "a mark of shame." Leonardo tosses the pocketknife at Cha's feet and disappears into the shadows telling Cha to "wear it well."
Cha is later shown at his wife's grave eating an apple with the pocketknife and claiming that Leonardo is wrong: the scar is a mark of honor, not shame.
Cha continues working with the Foot gaining an elite position in the organization. Cha can no longer openly wage his hatred war against Leonardo under the Foot banner due to the truce between the Foot and the turtles, but he, nevertheless, sabotages many of Leonardo's efforts.
At one point, Cha and Leonardo spar once again, but this time with wooden training swords. The fight lasts for quite awhile until, finally, Leonardo gains the upper hand and traces the scar with the wooden sword.