Seiichirou Shingyouji is portrayed as stoic and reserved, often keeping his thoughts to himself. He tends to express his opinions more openly when around his close friends, particularly Ibuki Ueoka and Natsusa Yuzuki, and occasionally teases Natsusa in a lighthearted manner.

Seiichirou shows deep concern for Natsusa, frequently visiting him and offering support after his injury. Although he worries about Natsusa pushing himself too far, he remains by his side to provide encouragement whenever needed. Their bond is highlighted by Seiichirou’s willingness to quit rugby when Natsusa could no longer play, though he ultimately continued after Natsusa expressed that he enjoyed watching him score.

While generally calm, Seiichirou’s strong sense of loyalty can reveal a more intense side to his personality. When Ibuki confides that he broke a significant promise, Seiichirou remarks that if Natsusa were to do the same, “even an apology would not be enough,” surprising those around him with the depth of his attachment. Beyond his stoicism, he places great importance on trust, promises, and the relationships he values most.

A rare glimpse of his softer side is shown in the final episode, when he carries Natsusa and asks if he is happy. After Natsusa responds affirmatively, Seiichirou smiles, one of the few times he displays genuine outward emotion.

Seiichirou Shingyouji is depicted as a young man with a tall and athletic build, suited to his position as a rugby fullback. He has short black hair and striking red eyes, features that give him a composed and serious look in line with his reserved personality. Like his teammates, he wears the Doshisha University rugby uniform during matches, while his casual attire is shown to be simple and practical, reflecting his straightforward nature.

Seiichirou Shingyouji grew up alongside Natsusa Yuzuki and Ibuki Ueoka, forming a close-knit friendship with them from childhood. Among the group, he is especially devoted to Natsusa, often acting as his caretaker and source of support.

Following Natsusa’s accident, Seiichirou frequently looked after him, even carrying his belongings on his first day back at school. Concerned for his well-being, Seiichirou expressed worry that Natsusa might overexert himself in his role as team manager. Despite occasionally disagreeing with Natsusa’s decisions, he consistently respected them and offered his support. His protective nature toward Natsusa is a recurring element of his character.

Seiichirou’s rugby career is also closely tied to Natsusa’s influence. He began playing the sport because of him, and his performance on the field often reflects their bond. Natsusa once noted that Seiichirou rarely missed goals after his second year of high school, crediting his calmness under pressure. However, Seiichirou later revealed that his nervousness in certain games stemmed from the fear of disappointing Natsusa if he failed to score.

Although Seiichirou appears calm and collected, many find his emotions difficult to read; even Ibuki, his childhood friend, struggles to fully understand him. Natsusa, however, is able to interpret his feelings with ease. While generally composed, Seiichirou is easily unsettled when situations involve Natsusa, something that teammates such as Madoka Hongou often notice and playfully provoke.

Natsusa Yuzuki
Seiichirou shares the closest bond with Natsusa, having been his friend since childhood. Deeply protective of him, Seiichirou often worries about Natsusa’s health and well-being following his accident, sometimes carrying his belongings or cautioning him against overexertion. Their connection runs so deep that Seiichirou once considered quitting rugby when Natsusa could no longer play, only continuing because Natsusa encouraged him to keep scoring. He is one of the few people able to clearly read Seiichirou’s emotions, and their relationship is central to his character development.

Ibuki Ueoka
As another childhood friend, Ibuki shares a long history with Seiichirou. While they maintain a supportive friendship, Ibuki often finds it difficult to fully understand Seiichirou’s feelings, a contrast to Natsusa’s perceptiveness.

Madoka Hongou
Seiichirou’s composed personality often makes him a target of Madoka’s playful teasing. Madoka enjoys provoking Seiichirou, especially when it involves Natsusa, though these interactions also highlight Seiichirou’s underlying emotional investment in his relationships.

• The given name Seiichirou (清一郎) is composed of sei (清), meaning “clear,” “pure,” or “clean”; ichi (一), meaning “one”; and rō (郎), a common masculine suffix meaning “son” or “man.” Together, the name can be interpreted as “pure first son” or “one who is clear and upright.”
• His surname Shingyouji (真行寺) combines shin (真), meaning “real” or “genuine”; gyō (行), meaning “line,” “row,” or “verse”; and ji (寺), meaning “temple.” The surname may be read as “temple of true practice” or “genuine line temple,” reflecting imagery of sincerity and structure.