Zeno, also known as Zeno the Undefeatable, is a large Triceraton who was imprisoned by his Empire for speaking out against Emperor Zanmoran, which was considered treason. He is now a trusted ally of the Ninja Turtles.
Zeno is a self-proclaimed traitor to the Triceraton Empire. He was once a respected Triceraton warrior, but he disagreed with Emperor Zanmoran's plan to find the Heart of Darkness. He believed that there is no honor in a weapon that destroys worlds from afar, even if it is against the Triceratons' sworn enemy, the Kraang. For saying such things, he was thrown into a pit and became a slave. They only let him out to fight gladiator matches to the death in the Tri-Arena, for the entertainment of the entire Empire. After defeating countless enemies, he developed a reputation as "the Undefeatable".
He is first seen killing another alien in the Tri-Arena as the spectating Empire cheered on. The Turtles, who were just captured while trying to infiltrate the Triceraton flagship, also witnessed this before being imprisoned at the order of Zanmoran. Some time after, the Turtles were thrown into a cell next to Zeno's. They could not see each other face to face and only could talk to them through a small opening at the bottom of their cells. Upon hearing their attempts to escape their cell, Zeno tells them not to waste their time, as there is no escape from the Tri-Arena. When the Turtles are brought food, Michelangelo slides Zeno some food, which Zeno thanks him for. When Mikey starts pondering aloud what he will fight, Zeno tells them that newcomers always fight a Spasmosaur and said that they will meet a quick and painful doom. Despite this, he tells them the creature's one weak spot: a small white spot on its flank. He says it is hopeless to reach, but wishes them luck as they are called for their next battle. Before leaving, Mikey ask his name, and is told that he is called Zeno. After the Turtles succeed and are thrown back into their cell, Zeno congratulates them on surviving. Michelangelo then asked him how he ended up in the pit, and Zeno told him his story. The Turtles are called up again for another fight. Zeno tells them he fears he knows who they are fighting next. Leonardo told Zeno that if they never see him again, they wish him luck getting his freedom. Zeno thanks them, saying that he will need it. When the Turtles reach the arena, they are shocked to discover that they were pit against their friend Zeno, who was also the same massively sized Triceraton who they witnessed kill the alien they saw earlier. The Turtles were hesitant to fight him due to two things: 1: Because he was their friend, and 2: Because they would certainly die in the arena against him. Zeno felt sympathy for the Turtles, but decided that he must fight, and told them that he will give them as honorable a death as possible. The fight was no challenge at all as the Turtles proceeded to be savagely beaten by Zeno. As he pins Leo down and was about to kill him with his mallet, Donatello, Raphael, and Michelangelo begged him to stop. They told him that they wanted to destroy the Black Hole Generator, and that their planet was at stake. In surrender, the Turtles drop their weapons as Zanmoran orders him to finish them in order to regain his freedom. To the surprise of everyone, Zeno chucks his mallet at Zanmoran's throne, nearly hitting him. Zeno then tells Zanmoran that the Turtles are honorable beings because they want to destroy the Heart of Darkness, but he would use it like a cowardly Kraang. Angered, Zanmoran orders his troops to destroy him and the Turtles. Zeno orders the Turtles to find the Black Hole Generator and destroy it, as he deals with the troops. The Turtles reach Emperor Zanmoran's throne and take him hostage to ensure Admiral Mozar would allow their escape. Mikey asked if they should help Zeno, but judging by how he was easily beating many other Triceratons, Donnie said that he can take care of himself. After escaping to the Ulixes, Mikey asserts that they never would have made it out without Zeno.
On the surface Zeno is a blood thirsty warrior, reveling in the victories he faces in the arena. But as he reveals to the Turtles, he is a Triceraton who puts honor above all else, who prefers to fight enemies head on rather than attack them from afar which he feels is cowardly.
Perhaps the most apparent physical trait Zeno possesses is his immense size. He towers over other Triceratons, being roughly double the size to others of his species.
- Zeno strongly resembles Traximus from the 2003 TV series, but has a different name. According to Peter DiCicco, he is meant to be a different character from Traximus.
- It is unknown what happened to Zeno after the events of "The Arena of Carnage".