For those of you unfamiliar with Sharon and Rusty, they are the lead characters in Major Crimes, a sequel crime drama to the hit program, The Closer. Sharon Raydor is captain of the Major Crimes division of the LAPD. Rusty is a witness to a crime, and since he's under-age and his parents are not in the picture, Sharon is his guardian. The two shows intersect because the crime Rusty witnessed happened at the end of The Closer, a crime committed by the ultra-villain Phillip Stroh (who I literally hate with each and every breath). Rusty was . . . hustling at the time. Because his mother abandoned him when he was 15, and he didn't know what to do, he turned to selling himself to men so he could survive. I'm not condoning or excusing his behavior, but that's what he did because he felt he couldn't trust anyone to help him.
A lot of the personal focus in Major Crimes is helping Rusty heal from his experience on the streets and prepare to appear in court for Phillip Stroh's trial. He builds relationships, slowly and through much mistrust, with all the members of the division, all of them familiar characters from The Closer. Sharon was under no obligation to keep Rusty. They could have placed him in witness protection or sent him to foster care, or any number of things, but she decided to keep him instead. The question is why.