Daichi Saitou serves as a bodyguard for Shiho Kamakura, specifically assigned to protect Yuuta Shindou from physical harm inflicted by Shiho. He possesses stark white hair, contrasting its original brown color; while some attribute this change to childhood trauma from witnessing Sayuri Muranushi's smile, he maintains it is simply dyed. On duty, he consistently wears a formal black suit, white shirt, and black necktie.

He attended elementary school alongside Sayuri Muranushi and Kisaki Kondou. During this period, he developed a crush on Muranushi that expressed itself through bullying behavior toward her, a past he does not deny when confronted. These childhood experiences, particularly related to Muranushi's smile, resulted in a lasting sensitivity. This allows him to immediately recognize a similar unsettling "aura" in Masahiro Adachi upon their first meeting, concluding Adachi must have also witnessed Muranushi's smile.

Professionally dedicated to his bodyguard role, Daichi nevertheless exhibits a tendency to flee situations when frustrated or overwhelmed. He holds respect for Tasaka as a senior colleague. Despite his position, he displays minimal respect toward his employer, Shiho. His relationship with Muranushi remains complicated due to their shared history.