Jimmy was Mii's master.
During Mii's life as a pet she got along well with Jimmy, her child master. One day, bullies attacked Jimmy and Mii fought them off but sustained fatal injuries in the process.
Jimmy is a character in the Galaxy Express 999 series, which was created by Leiji Matsumoto, who is popular for the Captain Harlock franchise and its related series. He makes his first appearance in Episode #37 "Mii-kun's Mansion of Life". In the anime series, his design and creation are credited to Shingo Arakiand Shigeru Kogawa respectively.