Shuntarō Sako is a third-year student in Tama University's School of Literature and president of the Street Observation Research Society, a club dedicated to observing people in public. He has reportedly remained in his third year since Raika Oda enrolled.

Sako possesses thick glasses and grey hair. He openly identifies as a lolicon, demonstrating a pronounced interest in young girls. This manifests in his focused attention on Yuuta Segawa's nieces, Miu and Hina, while dismissing the 14-year-old Sora as "too old," mockingly calling her "oba-chan" (old lady). He justifies his preferences with the phrase, "Love is justice. Love is the strongest," a sentiment Raika later echoes, though she frequently disciplines his excessive behavior with a paper fan.

As club president, Sako devises unconventional activities. His eccentric leadership and behavioral tendencies contribute to the club's small size. Yuuta Segawa actively tries to shield his nieces from Sako's attention, particularly after learning of his pedophilic inclinations. Raika Oda, the club's sole female member, routinely intervenes to curb his inappropriate conduct, often using her paper fan. Despite his unsettling interests, Sako is portrayed as fundamentally non-threatening.

Sako maintains a friendship with Yuuta Segawa and Kouichi Nimura, frequently visiting Yuuta's apartment with other club members. Kouichi assists Yuuta in limiting Sako's access to the younger girls. While his interest in the nieces causes tension, Sako demonstrates some redeeming qualities through his dedication to club activities and occasional willingness to assist others, operating within his established behavioral patterns. His interactions remain consistent, primarily defined by his fixation on young girls, his leadership of the Street Observation Research Society, and his resulting dynamics with Yuuta, Raika, and the Takanashi sisters.