"He's gonna hurt me so bad."
Years prior to Legacy, Steven personally knew Charles, as his family worked for Charles's. When Charles inherited his late father's meat-packing plant, he somehow convinced Foster to aid him in his serial killing, getting him to abduct potential victims, sedate them, and take them to the plant, where Charles would torture and murder them. All the while, he psychologically abused him.
In the episode, after assisting Charles in killing a total of 64 victims, Steven is first seen picking up a prostitute named Maggie, sedating her off-screen, and taking her to Charles's plant, where she would be tortured by him. Steven later attempts to abduct a homeless woman named Mona, but the BAU, who were personally called in by Detective Cal McGee, spot him and he is apprehended by Morgan when he attempts to flee. The BAU eventually extract information about Charles from Steven after promising him protection from the former, and they track him down and shoot him dead before he could kill Maggie.
Steven would approach random prostitutes, junkies, and vagrants on the streets and get them into his car, presumably with some ruse such as sex, drugs, or a place to live. He would also sometimes pose as a Social Security worker in order to lure in some of his victims. He would then sedate them and take them to Charles's plant, where they would be tortured and killed by him.
No official profile was made of Steven, as the BAU were more focused on Charles.