He was voiced by Fred Tetasciore.
As Ivan Steranko
Steranko wears a black suit with shoulder pads with spikes on it, a black dress shirt, a red tie, an Ushanka with a yellow star on it, and two golden knuckle dusters on his fists, with a hammer on one and a sickle on the other. He is blonde-haired and has a diamond eye, as well as several scars on his face.
He is a mutant human-rhinoceros hybrid, with grey, thick skin and a large horn. His knuckle dusters seemed to have fused with his skin, and now wears army pants and a yellow shirt, with the same spiked shoulder guards he used to wear.
He is first seen talking to Shredder about a weapons deal. He seems close, as he gives an offhanded mention that they're "old friends." He is presumably knocked unconscious by the weapon that was meant to incapacitate the Shredder: this begins the slow descent of Shredder becoming distrustful of Steranko.
In "The Legend of the Kuro Kabuto", he was revealed to have some past with Anton Zeck, who (allegedly, accidentally), is the one who shot out Steranko's eye. Steranko, an ancient artifacts collector, hires Zeck in to steal the Kuro Kabuto from the Shredder. Among Steranko's ancient armory are the Spear of Destiny, Excalibur, and the armor of Alexander the Great. Steranko gets furious when Zeck fails.
In Serpent Hunt, Steranko is stuck in New York with Zeck because of the Kraang invasion. He then decides to capture Karai, Shredder's adopted daughter who has become a mutant snake. After a confrontation with the turtles, the successful duo capture Karai and made a contract with Shredder so that he and Zeck could leave the city. In the end the Turtles release Karai and Steranko and Zeck are captured by Shredder and his henchmen. As punishment, Shredder decides to make mutate Zeck and Steranko. Zeck is mutated first, being transformed into a hideous warthog-human hybrid to Steranko's horror. Ivan tries to convince his old friend not to mutate him as they have known eachother for ten years and he would be more useful to him as a human. Shredder isn't convinced, however, and has him pushed into the mutagen vat, tranforming him into a rhinoceros.
In The Pig and The Rhino, which takes place immediately after Serpent Hunt, he and Zeck begin the episode by lamenting the fact they've been turned into "freaks", followed by them attacking Shredder because of the pain and fear of what they've become. Shredder effortlessly defeats the both of them by himself and offers them to either serve him or die, both of them agreeing to serve him. Shredder requests them to find Karai so that Baxter Stockman can attempt to mind control her. On their way, they figure that it's the Turtles' fault that they've mutated, and they should take it out on them. Speaking of the Turtles, they, along with Casey Jones and April O'Neill, are also looking for Karai, who said "comet" before leaving, tasking themselves to search three places. While Zeck finds, attacks and captures Donatello and April, Steranko attacks Casey and Raphael, who, even with their best efforts, can't defeat the super-powered mutant. He is informed by Zeck that the rest of the Turtles are at Coney Island, so they both head there. At the park, Leonardo and Michelangelo have found Karai and managed to calm her down, only for her to be scared off by Zeck and Steranko, who threaten to kill their allies captured in their van. While Leo stealthily picks the lock to the van, Michelangelo distracts the duo by trying to come up with a set of cool names for them, like he usually does in the show. He goes over a couple, until he decides Zeck should be named Bebop and Steranko Rocksteady. Rocksteady really likes his new name and uses it to refer to himself, while Bebop hates his. They are defeated by all the Turtles, with help from Casey, April and Karai. When Karai is trying to scavenge food after she ran away to not endanger the Turtles because of her slowly losing her mind from the mutagen, she gets trapped in a cage by Bebop and Rocksteady, showing they did succeed in their original mission.
In the Noxious Avenger, Bebop and Rocksteady battle the turtles while collecting chemicals to create Shredder's brain worms and accidentally drop a canister of mutagen on local sewer worker, Garson Grunge, mutating him in to Muck-Man. Fed up with being bossed around by Shredder and his henchmen, the duo manipulate Garson into thinking the turtles mutated him and use him to carry out their own nefarious deeds, until the truth is revealed and they are defeated.
In Attack of the Mega Shredder, after Leonardo and Michelangelo are captured by the Foot Clan, Bebop and Rocksteady watch them while Shredder and his minions are preparing to "double-mutate" them. Afterwards, the Turtles trick Bebop and Rocksteady into throwing the three Shredder mutants into the vat, planning to combine them with the turtles to create a giant super mutant, but instead create the titular Mega Shredder. They're beat up by Shredder's lackey Tiger Claw as punishment.
In Annihilation Earth Part 2, after Splinter and April infiltrate Shredder's lair and get him to agree to helping them fight off the Triceratons, Rocksteady join him and his other minions when they decide to team up. Along with everyone excluding the turtles, April and Casey Jones, Bebop and Rocksteady are both killed when Shredder (quite literally) back stabs everyone by killing Splinter, preventing him from stopping the Black Hole Generator. However, they are saved when the six survivors and the alien cyborg, the Fugitoid, time travel and prevent Shredder from killing Splinter and the Black Hole Generator from going off.
In City at War, Bebop and Rocksteady are seen guarding the now sick and dying Shredder at the end of the episode.
In the next episode, Broken Foot, they accompany Tiger Claw and help him protect the Alumon Chemicals factory and the Foot-Bot assembly factory from Karai, Shinigami, Leonardo and the other turtles.
In The Super Shredder, Rocksteady burst in to Stockman's lab when he hears Shredder screaming, only to become horrified when he sees him mutated into his new Super Shredder form. He and Bebop, along with multiple Elite-Foot Bots, appear at the end of the episode and burst through the wall in front of the Undercity, encountering the Turtles, April and Karai.
In Darkest Plight, which takes place directly after the last episode, they are seen fighting the Turtles, April and Karai. After bursting through the wall, they argue about weather they should squish or blast the Turtles and are then easily tricked by Michelangelo, who delays them long enough for Raphael and Donatello to sneak up and defeat them. After climbing out of the chasm in the Undercity, Super Shredder orders them to search the chasm for Splinter, believing that he may still be alive. Bebop and Rocksteady initially argue over who will go first, but they then decide to simply ambush the Turtles after they have found Splinter and Bebop uses his suit to make them invisible. However, one of Casey Jones' hockey pucks ends up hitting the invisible Bebop right in his snout, blowing their cover. The two mutants then engage Raphael and Casey in a fight and end up losing due to their incompetence.
In Requiem, Bebop and Rocksteady along with Rahzar and Fishface help Shredder's master plan in distracting the Turtles and their allies by going to the same amusement park the Turtles encountered them first. They've hooked up missiles to a merry-go-round and, along with the weaponized ride, attack the turtles. While they are beaten (and Rahzar is even drowned by Leatherhead), they are mysteriously cheery. The Turtles find out about their plan being a mere distraction and hurry to the Wolf Hotel, only to see the distraction being succesful and Splinter having been killed by Super Shredder.
In Owari, Bebop and Rocksteady have another geared-up dojo with all kinds of traps inside it. It's quickly hacked by Donatello, however, and the two end up fleeing after they're beaten.
In Wanted: Bebop and Rocksteady, it is shown in his resumé that he fought as a volunteer in the wars in Iraq, Yugoslavia and the DR Congo before he joined the Foot. He joins the classic '87 versions of Shredder and Krang who want to hire him and Bebop after they accidentally left the '87 versions of Bebop and Rocksteady behind. Much like the Turtles did in Transdimensional Turtles, they question some of the logic of the '87 series. When Shredder and Krang threaten to destroy the world, Bebop convinces him to stop them so "Mama" (His home country, Russia) will not be destroyed.
While he and Anton Zeck had an uneasy alliance (especially on Zeck's part) in The Legend of the Kuro Kabuto, they ironically became much better friends after they were mutated due to both of them not wanting to turn into mutants. This is shown by Steranko acting a bit more silly like Bebop does and even enjoying his mixtape. They also seem to hug and comfort each other multiple times, though in a comedic way.
- Michelangelo gave him the name of Rocksteady. The name has much more to Ivan who even made reference to himself as Rocksteady.