Mayumi is the name of a C-14 rifle.
Chun "Big Dog" Leung was a marine in Dominion section, 33rd Ground Assault Division. It was led by Arcturus Mengsk. Leung took very good care of his rifle, naming it Mayumi. He refused to allow other troopers to touch it.
At the Battle of Noranda Glacier, Mengsk was knocked down by enemy fire, and when Leung pulled him up, he himself was shot down. With his last strength, Leung told Mengsk to take care of his rifle, then expired.
Mengsk tried his best to keep the weapon clean after he left the military and became a prospector, but couldn't do as thorough a job as Leung.