Tsumugu is a tall and handsome young man with chin-length dark brown hair, swept to the side with bangs covering part of his forehead. His eyes are a muted puce color, giving him a composed and seemingly unreadable expression. His features are calm and refined, reflecting his quiet nature.

He is usually seen wearing his school uniform during the early part of the series.

After the Timeskip
Following the five-year timeskip, Tsumugu adopts a more casual style, often wearing a red sweater layered over a black shirt, accompanied by black jeans and blue shoes. His overall appearance remains largely unchanged, but he carries himself with more maturity and emotional clarity.

Tsumugu is a calm and composed individual, often perceived as stoic due to his reserved nature and tendency to conceal his emotions. He speaks in a straightforward and honest manner, which can come across as blunt, but is rarely meant with ill intent. His quiet attitude is complemented by a strong sense of intuition and emotional intelligence, making him a perceptive and thoughtful presence among his peers.

Despite his reticent personality, Tsumugu is empathetic and observant, often choosing to listen rather than speak. He is known to keep the confidences of others, particularly those of Chisaki Hiradaira and Manaka Mukaido. However, after the timeskip, he becomes more open, especially as Chisaki begins to understand her own feelings, including those directed at him.

Tsumugu once confided in Miuna Shiodome that he envied Hikari Sakishima and the others for their ability to express emotions freely something he himself struggles with. This introspective moment highlights his internal conflict and his quiet desire to better understand and communicate his own feelings. Miuna, in turn, admits she sometimes wishes she could hide her emotions more easily, offering a subtle contrast between them.

Tsumugu moved to the seaside town to live with his grandfather when he was nine years old, leaving behind his life in the city. He later confides in Chisaki Hiradaira that his parents held a dislike for the sea and chose to distance themselves from his grandfather, effectively abandoning him. Tsumugu maintains a close and respectful bond with his grandfather, often showing his appreciation in quiet ways. On one occasion, he gave his grandfather a gift of fish scales, a gesture full of meaning to Tsumugu. Although his grandfather didn’t grasp the symbolism, his acceptance of the gift brought Tsumugu comfort. In contrast, Tsumugu’s relationship with his mother appears strained, as he sometimes goes out of his way to avoid even speaking with her, using his friends as a buffer during her visits.

Chisaki Hiradaira
Before the timeskip, Tsumugu and Chisaki shared a complicated relationship. He was aware of her feelings for Hikari and often gave her advice on how to manage them. Their interactions were tense at times, marked by quiet emotional distance. However, over the five years that passed, their relationship gradually softened. After the timeskip, Tsumugu confides in Kaname that he has developed feelings for Chisaki, but chose not to act on them until their friends from Shioshishio had awakened, fearing that their return would take Chisaki away from him.

When Hikari later asks Tsumugu to feign romantic interest in Manaka to help her regain her lost emotions, Tsumugu refuses. He reveals that he cannot fabricate love and admits that he’s in love with Chisaki, a confession that both surprises Hikari and is overheard by Chisaki herself. Distressed, she runs into the sea, and Tsumugu follows, risking his life to reach her. Gaining ena for the first time, he follows her into Shioshishio to speak openly with her. He shares how much he has noticed and cared for her over the years the way she changed, laughed, cried, and carried herself and expresses that he fell in love with her through all those moments.

Initially, Chisaki rejects his confession, believing Manaka harbored feelings for him and feeling conflicted about her own past feelings for Hikari. Tsumugu corrects the misunderstanding by telling her that Manaka has always loved Hikari. Overwhelmed, Chisaki admits she was trying to hold onto the past but now understands that her heart has changed. When Tsumugu presses her to acknowledge her feelings, she finally stops denying them and confesses her love for him. Though hesitant at first, she allows herself to find happiness with Tsumugu, and the two become a couple.

Manaka Mukaido
Tsumugu cared for Manaka as a friend and was never romantically interested in her. His initial fascination with her stemmed from her ena and her origins beneath the sea. Manaka, in turn, opened up to him, sharing stories and confiding in him, especially during emotional moments like her fights with Chisaki. While Manaka believed she might have feelings for Tsumugu, she later realized her heart belonged to Hikari. She entrusted Tsumugu with her secret, and he quietly respected it. However, when Chisaki rejected Tsumugu under the false belief that Manaka loved him, he was forced to reveal the truth that Manaka’s love was always for Hikari helping resolve misunderstandings and allowing everyone to move forward.

Kaname Isaki
Tsumugu and Kaname share a respectful, if complicated, connection. During a near-drowning incident, Kaname saved Tsumugu despite momentarily considering letting him die out of jealousy for Chisaki’s affection. Tsumugu later thanked him for saving his life. As Kaname developed feelings for Sayu, he helped Tsumugu understand the reasons behind Chisaki’s initial rejection and encouraged him to clear up the confusion, leading Chisaki to accept her feelings for Tsumugu.

Hikari Sakishima
At first, Hikari disliked Tsumugu, believing Manaka had developed feelings for him after being rescued from a fishing net. Over time, as Hikari matured and better understood both surface and sea dwellers, he grew to accept Tsumugu. Despite their rocky start, the two formed a friendship, and Tsumugu quietly watched over Hikari, offering unspoken support and understanding when needed.