Archimedes McPatrick was born in 1967 in Ireland, he was better at hunting than his brother Brandon, he and Shredder have been business partners in the mafia for 10 years and he's got an emerald for a right eye because Anton Zeck shot the real eye out by accident.
McPatrick planned on hunting Karai as a means of trading her to Shredder for a way out of the Kraang-invaded city, but when the Turtles get in the way, both he and Zeck are captured by Rahzar and Tokka and tossed into a computer-based vat of Mutagen in Stockman-Fly's lab. McPatrick, having been infused with white rhinoceros DNA, mutates into an 8-foot-tall humanoid white rhino with a jagged horn ripping out of his droopy beak-like nose and his shoulder pads and knuckle dusters fused to his skin. He lands right next to Zeck and complaines that he doesn't feel so well.
As Archimedes McPatrick: McPatrick is large, and muscular and wears a dark green suit with spiked shoulder pads, a black undershirt, a green tie, a beret with a green clover, and a pair of knuckle dusters (with the Irish symbol on them). He has orange hair and mustache and an emerald right eye, as well as several scars on his face.
As Rocksteady: Rocksteady has a jagged horn ripping out of his hook-lipped nose and wears a black tank top, green bandoliers, and camouflage pants. His shoulder pads and knuckle dusters are fused to his grey skin.
Archimedes McPatrick is shown to not care about others, but about rare and personal items, making him selfish and vain, as he is shown to be quite a coward using Karai to attack Rahzar, he is also shown to be aggressive and not so bright.
As Rocksteady, he is even more aggressive and develops more brave skills to suit his new form.