Cosmo Brown is a fictional character from the classic 1952 musical film, Singin' in the Rain. He is portrayed by actor Donald O'Connor and serves as the best friend and sidekick of the main character, Don Lockwood. Cosmo is known for his comedic talent, charm, and energetic personality, often providing comic relief throughout the film. In addition to his role as a friend, Cosmo is also a talented performer and songwriter. He plays a crucial part in helping Don transition from silent films to the new era of talkies. His famous performance of the song "Make 'Em Laugh" showcases his exceptional dancing and singing abilities, solidifying his status as a beloved character in musical cinema.