|Name in Japanese||永澄の祖母|
|Rōmaji Translation||Nagasumi no Sobo|
|Voice Actor (Japanese)||Eriko Matsushima|
|Voice Actor (English)||Juli Erickson|
Nagasumi's grandmother (永澄の祖母 Nagasumi no Sobo ) makes an appearance early in the story and allows Nagasumi to stay at her house during their stay with her.
In the first episode, The Yakuza Wife, Nagasumi's grandmother was the first of his family to believe him when he claims a mermaid saved him from drowning. Also, when Sun Seto showed up, Nagasumi's grandmother was completely unphased by what the mermaid said, as opposed to his parents' aggressiveness towards Nagasumi due to misinterpreting Sun's meaning when she asked for him to "take responsibility for what happened between them [earlier that day]."
In short, she not only took Nagasumi's claims seriously, but she was unsurprised by the Seto Group coming to bring her grandson and his parents back to the lake. This suggests, but does not confirm, that she was somehow aware of merfolk to the point of believing the legends. Seeing as Nagasumi is not part merfolk, it is unknown how his grandmother knew about the merfolk before the events of the first episode.