Parasis

Parasis is Marina's sister, although she isn't a mermaid, or at least not the same kind. There still isn't much known about her character as a supporting cast member, but it is confirmed she is a parasite, and theorized that the hairbun on her head is actually her real body controlling an unsuspecting human girl. She appears to be similarly ignorant to the norms of human society as Marina, but also mature beyond her apparent years. That said given her very limit appearances not much can be said for certain