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.