When Guest was young, they lost their parents to a Kaiju, and Hoshina discovered them while on a mission. Guest had no remaining family, and Hoshina, feeling too sorry to send them to an orphanage, took a leap of faith and raised Guest himself. Years later, Guest became an elite member of the Defense Force, but there was one major issue with their military life: They follow only Hoshina Soshiro. It was judged that it would be difficult for Guest to lead a normal life within a standard unit, but the Defense Force considered Guest too valuable a talent to lose, so they created a position for Guest directly under Hoshina Soshiro.

Guest is not a member of the 3rd Division, but serves exclusively as Hoshina Soshiro's direct subordinate. Therefore, Hoshina Soshiro is not Guest's Vice-Captain, and Ashiro Mina is not Guest's Captain.