Being a clone of Leonardo, Dark Leo is the leader of the Dark Turtles.
In his first appearance during, Leo tries to talk to him, saying he has to have honor if he's truly his clone. Dark Leo himself admits that he and the other Dark Turtles have many similarities to Leo and his brothers, but says honor and value aren't one of them.
The next time he and his brethren are seen is in The Fall of Darius Dun where he ends up having to face all four of the Turtles when his brothers are defeated by the originals. He too is defeated, and is shown back in their hideout at the end where they have taken Darius Dun.
After failing to beat the Turtles, Dark Leo is pressured by Darius Dun to come up with a new way to defeat the Turtles (DNA is Thicker than Water). He comes up with the idea to attack the Turtles from inside, stating "If you can't beat them, join them." He and his siblings attack the O'Neil Penthouse in daylight, where Dark Don shoots a laser at Cody, but Dark Leo blocks it, claiming it is against the plan, and rescues Cody, physically damaging himself so much that he falls unconscious.
His brethren flee, leaving him behind only to be taken care of by the Turtles and Cody. They don't risk anything though and keep him in a force field laser grid in the medical bay.
During Dark Leo's stay, the Turtles and Cody decide to give Dark Leo a chance to become good. They allow him to eat breakfast with them all and he quickly devours most of the food in a disgusting manner, claiming he and his brothers have to eat when they can or they'll starve, upon which Dark Leo ends up learning a lesson about karma.
To see if he had recovered physically, Dark Leo was given a test in the Dojo where he had to lift a bar. He pushes on it and makes Don think he hasn't recovered enough, but then Raph heckles him to prove that the villainous terrapin has almost completely recovered from his injury. He breaks the machine in his rage and as it explodes, a piece nearly smashes into Raph, but Dark Leo smacks it out of the way, saving Raph. As he towers over Raph, snarling in his face, the laser grid is brought up around him so he can't hurt him. He then claims Raph thinks he's figured him out but he knows nothing about him.
One evening, Leo visits Dark Leo to tell him the story of the frog and the scorpion, allowing Dark Leo to think about the moral. Leo then gives him a bonsai tree and says they'll give him two options in the morning, go with the peace keepers or stay. As Leo leaves, he drops the force field controller behind him, which Dark Leo uses to escape and call on his brethren, revealing this was all part of his plan.
As he goes to turn off the security system so his siblings can get in, he begins to get second thoughts about his plan. Leo surprises him, telling Dark Leo he still can do the right thing by going back to the lab. But Dark Leo states he knows what he is and that he's a scorpion. He then turns off the security by slashing the switch, allowing the other Dark Turtles to enter.
After fighting with Leo, who still tries to convince him he has a choice in this, Dark Leo tells his brothers the plan is a bust and they all flee in the Terrorpin. When they return, Dark Leo faces a furious Darius Dun. After scolding him on his failure, Dun throws him a steak and states that he hopes he chokes on it. But Dark Leo rips the steak into four pieces and shares it with his brothers, confusing them thoroughly. He then takes out the bonsai tree that he had received from Leo while with the Turtles, symbolizing change and implying he learned something while living in the penthouse.