 Sean Maher as Dr. Simon Tam — a trauma surgeon of the first caliber ( top 3 % in his class at a top core @-@ planet institution ) , who is on the run after breaking his sister River out of a government research facility . In the episode " Safe " , it is revealed that he and River had a privileged upbringing with access to the best education . In rescuing River over his stern father 's severe objections , Simon sacrificed a highly successful future in medicine . His bumbling attempts at a romantic relationship with Kaylee are a recurring subplot throughout the series , and at every turn he seems to find a way to unwittingly foil his own attempts at romance . His life is defined by caring for his sister . 
