"You start out with whatever your f***ed up parents and genes give you. Then, if you see enough shrinks for long enough and get your cocktail right, you can get over yourself - and have a life."
Billy is Brenda's younger brother, a bright but disturbed young man. He's a diagnosed manic-depressive who, as Brenda says, "always seems to pick the holidays to go off his meds." In the past, Billy - who could be charming one moment and hostile the next - was highly possessive of Brenda, who seemed to have made taking care of him her life's work. As children the two of them were nearly inseparable, and as adults it was Brenda who Billy turned to when he felt overwhelmed. For example, when Billy, a gifted photographer, had a show at a local gallery, he couldn't select the photos himself - he had Brenda choose them. The reason for this, he told Nate was that, "sometimes she gives me her eyes because sometimes I go blind."
When Nate came into Brenda's life, Billy's jealousy and instability drove him to become genuinely dangerous. On one occasion he followed Brenda and Nate to Las Vegas, slipping into their hotel room and photographing them while they slept. Despite Brenda's assurances that Billy would never harm anyone, he broke into her house one night and attempted to cut a tattoo from her back with a mat knife. Immediately after that, Brenda did something she'd hoped she'd never have to: she had Billy committed to a mental health facility.
Billy's doctors got him back on his medication and, as he puts it, "pumped so much electricity through me it could light up the eastern seaboard." After several months he was released, and came to see that being committed to the "home for the tragically inappropriate" was the best thing that could have happened to him. On his therapist's advice, he disengaged for a time from his "toxic" relationship with his sister. When they eventually resumed seeing each other, Billy, once again emotionally strained, made an advance on Brenda and she fled his apartment, once again leaving Billy completely alone.
His father's death and his distance from Brenda have forced Billy to become more self-reliant, and after an art grant took him to Patagonia, he's back in LA substitute teaching at LAC-Arts. When his mother collapses and undergoes an emergency hysterectomy, Billy must face Brenda again in the hospital and they manage a civil lunch.
His new teaching job also reconnects Billy to Claire - who happens to be one of his students. No longer seeing her as Nate's kid sister, he coaches her through her first photo exhibit, and seems pleased when she invites him back to her place.