Jirō is a tall high school student with dark-blue hair styled in a short cut with bangs and distinctive red-purple eyes. He typically wears the standard school uniform, complete with a dark blue necktie that sets him apart from his peers, and outside of school he favors plain, monochrome t-shirts and shorts.

His personality in high school stands in stark contrast to the cheerful and sociable boy he was during grade school. A pivotal event in middle school shattered his self-esteem and reshaped his entire demeanor. When he attempted to confess his feelings to his childhood friend and crush, Shiori Sakurazaka, he misinterpreted her wish to "always be friends" as a rejection. This misunderstanding caused him to retreat from social interactions, becoming a meek, anxious, and reclusive individual who spends most of his free time playing video games alone or with his only close friend, Sadaharu Kamo. Jirō lacks self-confidence and harbors a significant inferiority complex, constantly comparing himself unfavorably to more popular and athletic boys like Minami Tenjin.

Despite his withdrawn nature, Jirō is not inherently lazy. He will complete tasks when asked, but he lacks proactivity, preferring the escapism of gaming over productive activities. This has earned him a reputation as a slacker among his classmates. His reclusiveness has also severely stunted his ability to interact with girls on a romantic level, and he has never had a girlfriend. Ironically, while Jirō fears being judged by others, he is prone to misjudging people himself, often jumping to incorrect conclusions about their motives based on his own anxieties.

At his core, however, his childhood kindness remains intact. He is genuinely concerned for the emotional well-being of his friends and has a habit of accepting blame unnecessarily to avoid conflict. This caring nature is what made Shiori fall in love with him and later becomes a source of emotional support for his practical partner, Akari. His desire to keep everyone happy can also be a hindrance, as it leads to indecisiveness in difficult situations where any choice might cause someone he cares about to suffer.

Jirō's primary role in the story is as a third-year high school student forced to participate in the school's "Marriage Practical," a course where male and female students are paired up to live together as a mock married couple. His initial and only goal is to earn a high enough score with his assigned partner to qualify for a partner shuffle, allowing him to finally be paired with Shiori. Unfortunately for him, he is randomly assigned to Akari Watanabe, a popular gyaru who is his complete opposite and who also wants to switch partners to be with her own crush, Minami. To make matters more tense, Shiori and Minami are partnered together.

His key relationships are the primary drivers of his character development. His relationship with Shiori is defined by mutual, unspoken feelings and the lingering awkwardness stemming from their middle school misunderstanding. His relationship with Akari begins with mutual friction and preconceived negative judgments but evolves into a deep and complicated reliance. Forced to live together, they push each other to grow. Akari's influence helps Jirō become less socially anxious, more proactive, and more confident. He starts exercising daily with her and vows to become more supportive after realizing she has been handling most of the household chores alone. Their accidental first kiss and subsequent intimate moments blur the lines of their pretend relationship, creating a profound internal conflict within him.

His friendship with Sadaharu provides him with a comfortable space to share his feelings and interests, though his friend's eccentric and perverse behavior often leaves him unsettled. His dynamic with Minami shifts from jealous rivalry to mutual respect after a pivotal event during the sports festival, where Jirō hesitantly agrees to replace an injured student in a relay race. This act not only helps his team win but also earns him recognition from Minami, helping to ease his long-held inferiority complex.

Jirō's development throughout the story is one of gradual maturation. He moves from being a withdrawn, indecisive, and self-loathing individual to someone who is more proactive and self-assured. A major turning point occurs when he finally recognizes that he has fallen in love with Akari, a realization that forces him to confront his lingering feelings for Shiori. However, even as he becomes more decisive, his inherent kindness and occasional indecisiveness persist, leaving him torn between the idealized love of his past and the genuine bond he has formed in the present.