Mikhail Diagilev serves as the chief engineer of the Cha-Cha Maru, the pet shop hunter vessel captained by Tita Mu Koshigaya. He is an older man, described as a senior male with short white hair and yellow eyes, and he holds the position of one of the most loyal and longest-serving crew members aboard the ship. His service on the Cha-Cha Maru predates Tita's captaincy, having originally worked under Tita's father, the previous owner. Eleven years before the events of the story, a catastrophic incident caused the ship's engineering section to be jettisoned; Mikhail carries deep guilt over that event, believing that he should have been the one to die instead of Tita's father. This guilt fuels his fierce dedication to Tita and his willingness to sacrifice himself to protect her or give the ship an advantage in battle.

Within the crew's family-like dynamic, Mikhail occupies the role of a gruff grandfather figure, contrasting with Balboa as the father figure and Mei as the mother figure. His gruff exterior masks a deep sense of responsibility and care for the younger captain. As a skilled engineer, he maintains the Cha-Cha Maru's critical systems, and his experience makes him an invaluable asset during dangerous missions. Mikhail's motivations are rooted in loyalty and redemption—he will do anything for Tita, even face death, to make up for his perceived failure and to ensure her survival. His notable abilities lie in his mechanical expertise and his steadfast resolve under pressure. Despite his rough demeanor, his unwavering support anchors the crew and underscores the found family theme at the heart of the story.