Dennis Kim is a 15 year-old physics prodigy from North Korea whom Dr. Eric Gablehauser hopes to recruit into his department's Ph.D. program, in the episode "The Jerusalem Duality" and also the main antagonist. He is portrayed by Austin Lee.
A highly sought after doctoral candidate, he was the valedictorian at Stanford University, graduating within a year-and-a-half, and the youngest recipient of the prestigious Stevenson Award (probably based on the real-life Stevenson Prize). Dennis lost a year in starting graduate work while his family tunneled out of North Korea, though his grandfather remained behind. As incentives to lure him, Dr. Gablehauser proffers that he may use up to 20% of the grant money he attracts to smuggle his grandfather out of Pyongyang and enlists Sheldon and Leonard to provide a tour of the department to show him why Caltech has the best physics research facility in the USA.
Dennis and Sheldon have an antagonistic relationship when the former intellectually surpasses the latter. Believing the string theory research (i.e., Sheldon's research) conducted at Caltech to be a dead end, Dennis insults Sheldon by insisting that he will eventually come to the same conclusion and inputs that the whole landscape of false vacuums in string theory could be as large as ten to the five-hundredth power. Dennis insults Leonard's English, before Sheldon halts the tour by returning to his office, Dennis makes a keen observation of Sheldon's work on quantum loop corrections scribbled on one of his whiteboards and finds it startling that he hasn't considered a Lorentz invariant field theory approach. While Sheldon demands that he leave, Dennis Kim excitedly takes note of the Stevenson Award framed near the door inside, but is quick to discover that Sheldon was only formerly the youngest recipient at fourteen-and-a-half.
Losing faith in his own research, Sheldon begins searching for a different field of study to the progressively increasing frustration of his friends, prompting the gang to come up with a solution to derail Dennis Kim's work. The gang make arrangements so Dennis can meet girls his age in an attempt to distract him from his intellectual pursuits. The plan somehow succeeds when Dennis finds himself a girl called Emma and begins a romantic relationship with her.
Later, The guys see Dennis in the park kissing Emma and drinking from a bagged bottle, presumably no longer an intellectual threat to Sheldon.