|Birthday||May 5 (Taurus)|
|Height||166cm (at 14 years old)|
|Occupation||Student, Fisherman, Sea Reasearcher|
|School||Mihama Middle School|
|Relatives||Isamu Kihara (Grandfather)
Chisaki Hiradaira (Girlfriend)
|Manga Debut||Chapter 1|
|Anime Debut||Episode 1|
|Japanese Voice||Kaito Ishikawa|
|English Voice||Chris Hackney|
A boy from the surface whose grandfather is a fisherman. One day, he meets Manaka Mukaido when she is caught in his net. He is quiet, stoic, and very interested in the underwater world. His grandfather was from the sea. Tsumugu has a complicated relationship with his mother. After the time skip, he attends college and majors in oceanography. He lives in the city and conducts research about the sea villages at his university. He becomes close to Chisaki Hiradaira and eventually falls in love with her. After accidentally claiming his love for Chisaki, he follows her into the sea and gains Ena. It is later revealed that he had known of Manaka's feelings for Hikari Sakishima since the eve of the Ofunehiki five years previously, but agreed to keep it a secret at Manaka's request. After having a talk with Kaname about Chisaki's true feelings, Chisaki finally accepts her love for Tsumugu and become a couple. He is last seen walking to the train station with Chisaki as he leaves for college.
Tsumugu is a handsome young man with long, dark brown hair that goes down to his chin. His eyes are puce-colored and rather emotionless. He is often seen with his school clothes. After the timeskip, he wears a casual outfit comprised of a red sweater on top of a black shirt paired with black jeans and blue shoes.
Tsumugu has a very calm demeanor and can even be seen as stoic. He rarely shows emotions on his face and instead, can be seen as blunt when speaking to someone. He is very intuitive though and mature for his age. He's soft spoken and understanding, preferring to listen rather than speak. He also mentions to Miuna Shiodome that he was envious of Hikari and the others because of their ability to express their feelings easily. This means that Tsumugu himself wishes he could express his feelings more, which can be a slightly mild form of alexithymia.
Tsumugu came to live with his grandfather when he left the city at the age of nine. He reveals to Chisaki that his parents disliked the sea, therefore abandoned his grandfather. He seems to have an uncomfortable relationship with his mother, as seen when he uses his friends to just avoid talking to her. He has a close relationship with his grandfather. He once gave his grandfather fish scales as a present and even though his grandfather did not understand the significance of the fish scales, his acceptance of them made Tsumugu happy.
On the morning of the sea kids' first day at Tsumugu's surface school, Manaka Mukaido is berated by Hikari Sakishima for wearing the uniform of her new school rather than their old one and decides to return home and change while her friends go on. After changing, Manaka returns to the meeting spot only to be fished up by Tsumugu and his grandfather.
During P.E Hikari races Tsumugu. Tsumugu, who had been running further ahead of everyone is oblivious to Hikari. However, while running, Hikari loses his balance and falls onto Tsumugu, taking both of them down.
The next day, as a group of girls take Manaka into the sun to inspect her supposedly shining skin, Tsumugu comes in, questioning the other girls about what they were doing. Manaka, flustered and embarrassed, makes a run for it. Tsumugu is left behind.
Tsumugu, unaware that the other sea kids have been looking for the missing Manaka, finds her unconscious on the forest floor. Knowing about the sea peoples' need for salt water, he brings her home. As he enter his home, his grandfather told him to put her in a bath tub. As Manaka awakens, Tsumugu is seen putting salt into her bath water. Manaka is shocked to see Tsumugu looking after her. Noticing her knee, Manaka asks him not to look, believing it grosses people out. Despite this, Tsumugu attempts to feed it, but is stopped by Manaka. When she continues to be embarrassed, he tells her that the fish was beautiful, the scales on it unique. He then adds that Manaka is also beautiful, complimenting her Ena.
That night, Manaka is taken by Tsumugu back to the water, where she spots Hikari searching for her. As Hikari attacks Tsumugu, Manaka breaks the two of them apart, telling Hikari that Tsumugu helped her. With the boys not fighting, Manaka and Hikari return to the sea, with Manaka looking after Tsumugu's retreating form.
Before the timeskip, Tsumugu had a rather complicated relationship with Chisaki. Tsumugu has known that Chisaki has a crush on Hikari, giving his own advice on ways of dealing with it. Often, they were at a standstill with a strained relationship but it had softened over from the timeskip. After the timeskip, Tsumugu confessed to Kaname that he has feelings for Chisaki but hadn't acted on those feelings until the remaining sea friends wake up so they can both move on. But it is revealed he was scared of them waking up because he thought he would lose Chisaki to them. Hikari then tries to force Tsumugu to pretend to love Manaka for the sake of getting her ability to fall in love back, but he then tells Hikari that he is in love with Chisaki, which she happens to hear. Shocked, Chisaki dives into the sea, where Tsumugu follows her and after believing he would die, he receives ena and follows Chisaki's trail to Shioshishio to talk to her. He tells her that she was always so quiet and calm, and yet sometimes she became so fierce that he couldn't handle her, just like the sea. Tsumugu tells Chisaki that the five years that they spent together, he was always watching her. He always came when she got angry, laughed, or cried and realizes that her feelings are for him now. Tsumugu then hugs Chisaki, but she breaks from his hug saying that she doesn't love him back. But after Tsumugu finds out the reason Chisaki rejected him was because she thought Manaka liked Tsugumu, Chisaki also learns that Manaka is actually in love with Hikari, she then admits that she is in love with Tsumugu. Since the two love each other, they become a couple.
Manaka use to love Tsumugu but, realize she loved Hikari instead. but she knows those were the old days. and tell Hikari her true feelings for him.
Kaname helped save Tsumugu when he drowned in the sea, but regrets it. When he was drowning, he thought to leave Tsumugu to die because then he could be with Chisaki. But, he saves him anyways and Tsumugu thanks him for that. Even if he didn't save him, he would never let go of Chisaki. After Kaname begins to fall in love with Sayu, he tells Tsumugu the reasons why Chisaki rejected him.
At first, Hikari hated Tsumugu, since he thought Manaka started having feelings for him after he reeled her in his net. He learns to accept Manaka's feelings for him and they both become friends.