A boy from the surface. At the start of the story, he accidentally fishes up Manaka and later befriends her. He's from a fisherman's family and likes the sea and the people living in it.
Tsumugu is a calm, collected, and solitary person. He is portrayed as being somewhere between merely stoic and matter-of-fact, rarely ever smiling or showing emotion at all. Tsumugu has a very inquiring mind when there is something he cares about, but at the same time, he had almost no tenacity of purpose, he prefers to observe then act. Despite his usually stoic demeanour, Tsumugu does have an emotional side, as he noted during his talk with Miuna, that he just doesn't know how to show it.
After 5 years, Tsumugu lives with Chisaki. He is now a oceanography researcher; he is researching about the sea villages and if the hibernation might happen on the surface.
It is hinted that Tsumugu may have feelings for Chisaki, but is too afraid to say anything because he believes Chisaki is still in love with Hikari. Tsumugu gains Ena after he dives after Chisaki, who escaped into the ocean when she overheard Tsumugu telling Hikari about his feelings for Chisaki. By the end of the series, Tsumugu and Chisaki are together.