Boom-Sook Kim is the graduate student Sonmi is forced to stay with after she's taken from Papa Song's. If you think your roommate is bad, be glad you don't live with Boom-Sook. Be glad you don't live in the same building as Boom-Sook.
When we first see him, he's singing "a popsong about Phnom Penh Girls" (5.1.138)—which must be Thailand's version of "California Gurls"—and we soon learn that his "concerns were not his Ph.D. but drinking, gambling, and his crossbow. [...] With such an upstrata father, study was a mere formality" (5.1.166). He's rich, spoiled, and selfish. A dangerous combination.
Oh, he's a prejudiced jerk, too, "address[ing] [Sonmi] like purebloods speak to a cat" (5.1.172). Not only does he disrespect fabricants, but he doesn't even see them as worthy of any kind of empathy. He laughs when news is delivered that Sonmi's fabricant friend, Wing-027, was fried to death in an accident. Boom-Sook was probably the kind of kid who tortured cats in his spare time.
Speaking of torture, when Dr. Mephi catches Boom-Sook about to shoot a plum off Sonmi's head with his crossbow, he instantly expels him, and we never see him again. Better late than never. Too bad his money and privilege will probably have him back on top and wrecking the lives of those on those at the bottom in no time.