Kashiwada's older brother is a college-aged young man who serves as a minor but memorable character in the story. He is the elder sibling of the protagonist Kashiwada and is known for his seemingly gentle demeanor that masks a sharp and protective nature. His most defining trait is his perpetual, unchanging smile, which remains fixed on his face regardless of the situation or his true emotions. This constant smile makes him appear calm and collected at first glance, but it also has an unsettling quality, as his facial expression rarely matches his internal state. In this way, he shares a similarity with his younger sister, who is known for her complete lack of facial expression. While Kashiwada's face remains blank, her brother's face is frozen in a pleasant but ultimately unreadable smile.

His personality is driven almost entirely by his devotion to his sister. He is extremely overprotective and is described as a chronic worrier who dotes on her above all else. This fierce protectiveness shapes his primary motivation in the story, which is to guard his sister from any perceived threats, particularly from boys. Because of this, he openly regards Oota, the male lead who is clearly interested in Kashiwada, as a rival and an enemy. He views Oota with deep suspicion and his unwavering smile hides a sharp vigilance towards him. His overprotective nature is so straightforward and intense that it has been described as refreshingly honest.

His role in the narrative is that of an obstacle and a source of comedic tension for Oota. He appears when Oota visits Kashiwada's home, and his mere presence, combined with his unsettling smile, is enough to deeply intimidate and frighten Oota, establishing him as a formidable figure that Oota must eventually contend with. Beyond this adversarial stance, his key relationships include his bond with his sister, which is the core of his character, and a possible connection to Oota's own older sister, as he is noted to attend the same university as her. This connection to the university suggests a life and social circle that exists outside of his sister's immediate world. As a supporting character, his development is limited, but he firmly establishes himself as a loving, if intimidating, sibling whose primary narrative function is to challenge the male lead. His notable ability lies in his unnerving, perpetually smiling facade, which makes his true feelings impossible to read and serves as an effective tool for expressing his silent disapproval and protectiveness.