Rusalka

The Goddess of Water, who has shorter blue hair, a young appearance, and a quiet demeanor. Due to Mukohda's lack of affinity for water magic, she grants Sui her blessing instead to receive offerings of otherworldly food. In the spin-off manga Sui's Great Adventure, she becomes attached to Sui and the two begin having long nighttime conversations together, with Sui eventually viewing Rusalka as an older sister.

(Source: Wikipedia)