Rocksteady was originally a Caucasian-American human member of a gang of criminals who operated under the Foot, when one of their thefts was interrupted by an attack by the Teenage Mutant Ninja Turtles. Rocksteady was originally a short and stocky blond Caucasian man (who sported army camouflage pants that would be replaced with simple beige cargo pants later while also occasionally sporting an army helmet on his head in his mutated form). With the other members of his gang, he was sent out to stop a Channel 6 reporter named April O'Neil from doing a report about crime in the city. April ran down into the sewers while being chased by the street gang and met the Teenage Mutant Ninja Turtles, who then defeated the gang in a fight.
After this humiliating setback, the Shredder developed a plan to defeat the Turtles by mutating members of the street gang, so that they would have abilities greater than the Turtles'. Rocksteady and his friend Bebop were voluntarily mutated into evolved animals (though neither was particularly aware of what it would entail) with the promise that it would allow them to exact revenge on the Turtles and gain superhuman powers. He was held down by belts on a laboratory chair, and his DNA was mutated with a rhinoceros kidnapped from the zoo. Rocksteady became a half-man, half-rhinoceros. As a result, Rocksteady had the enormous strength of his animal counterparts, with the disadvantage being his appearance. Together with Bebop, the second victim of the experiment, Rocksteady supported Shredder henceforth in carrying out his plans. However, though the transformation did make them larger and stronger, they remained incompetent and were often inept at stopping the Turtles or carrying out Shredder's plans.
For example, in Enter the Shredder they charged at the Turtles, who jumped, and crashed into each other. Donatello commented that their mutations didn't "up their IQ's any." For most of the series they were employed for important purposes, much to the annoyance of the Shredder when they eventually fail. However, the Turtles certainly consider them to be formidable (despite their stupidity) in combat due to their great strength and endurance, and as such, often use their intelligence to outwit them rather than fighting them in a straightforward manner. But their attempts at the turtles seem to regularly fail due to their incompetence and goofy behavior, which all leads to them being abused both physically and verbally by Shredder and Krang.
However, in season eight, Rocksteady along with Bebop seemed to have some form of intelligence, but talked and joked around less. Rocksteady and Bebop's last appearance is in the season 8 finale Turtle Trek. In that episode, the Turtles destroy the Technodrome's engines, whereafter a monster plant pulls the fortress into a giant pit. The impact of the fall presumably destroys the Technodrome's trans-dimensional portal (although it is shown to be still working in the series finale), thereby stranding its inhabitants in Dimension X for good. Rocksteady and Bebop are never seen again after that episode, although their bosses Krang and Shredder return to Earth during the 10th and final season. It can be assumed that he and Bebop either perished somehow or were left behind by their bosses. It is also possible that, much like in the comics (which were originally based on the events of this show), that they both found the Eden Worlds and contentedly lived out the rest of their lives as animals in its wilderness.
They made a reappearance in the made-for-TV movie 'Turtles Forever'. In the flashback describing how the Turtles crossed dimensions, they said to their Turtle counterparts that they were facing off against Shredder and the Technodrome, meaning that he got the machine out of Dimension X (as well as Bebop and Rocksteady). Their incompetence is still shown, although it ended up saving the Utrom Shredder when Rocksteady accidentally tripped over and unplugged a laser that was about to destroy him, although Bebop ended up obliterating the Utrom Shredder anyway when he replugged the same laser device all the while thinking he would be pleased that they "fixed" his machine. All this happened just as the Utrom Shredder was unleashing a plan that would wipe out Ninja Turtles of all planes of existence (even if it meant destroying himself since he was still linked to them), so ironically... Bebop saved all of the Turtles in existence.
Outside of that, Rocksteady's human form was seen in a street scene with the other gang members when the Turtles arrive in the 80's Turtles dimension.
2012 TV Series
In Wanted: Bebop and Rocksteady, Classic Rocksteady and his partner Bebop get left behind on accident when Classic Shredder and Krang use the Classic Technodrome to travel to the 2012 dimension. Shredder and Krang recruit the 2012 versions of Bebop and Rocksteady to do their bidding, dressing them up as their '87 counterparts. At the end of the arc, the '87 Turtles convince Bebop and Rocksteady to stop being Shredder and Krang's meat-shields. Rocksteady reveals that "he wants to be a personal injury lawyer".
- In Planet of the Turtleoids, Rocksteady states that the zoo they are robbing is the same one they came from when they were mutated. It is prominently known that they were both human before and not animals. It is possible that the memory comes from the animal DNA that mutated them, but it is unlikely.
- Just like Bebop, Rocksteady is named after a genre of music.
- In Muckman Messes Up, it is established that neither Rocksteady or Bebop can read.