Tomura is Erin's upperclassman at Kazalmu Beast Sanctuary, an institution training beast doctors. Respected by peers and underclassmen for his position and experience, he initially held responsibility for caring for Leelan, a young Royal Beast. When Erin, then a middle school student, was assigned to take over Leelan's care, Tomura's role diminished. Though this reassignment could have bred resentment, he instead grew intrigued by Erin's unconventional methods and unwavering dedication to understanding the beasts.

Observing Erin's deep bond with Leelan and her commitment to nurturing the Royal Beast's natural behaviors, Tomura developed admiration for her approach. This admiration gradually transformed into personal affection. He explicitly vowed to protect Erin from individuals or factions seeking to exploit her unique connection with Leelan for military purposes or political gain, particularly as tensions escalated between regional powers. His protective stance underscores his recognition of the value and vulnerability inherent in Erin's relationship with the beasts.