Shuntarō Sako is a third-year student at Tama University's School of Literature and President of the Street Observation Research Society, having held that class year since Raika Oda's enrollment. He is characterized as a lolicon and wears thick glasses. He once defended his lolicon interests to Raika by declaring, "Love is justice, Love is the strongest," a statement she later accepts, though she frequently disciplines his excessive behavior with a paper fan.

He exhibits a particular interest in young girls, often focusing on Yuuta Segawa's nieces, Miu and Hina Takanashi, while explicitly excluding Sora Takanashi, whom he considers too old despite being only 14. He frequently calls Sora "Oba-chan," which fuels her persistent dislike for him. Sako possesses the unusual ability to identify a girl's physical traits, such as age, eye color, and hair color, solely by hearing her voice.

Despite his eccentric and often inappropriate conduct, Sako proves to be a reliable friend. Along with other club members, he supports Yuuta Segawa in managing the household and caring for the three sisters. His actions, though frequently requiring intervention, ultimately contribute positively to the group's dynamic.