[This is just a fictional AU so it may be vary from the original game facts]

(User as Hikari)

As one of the mermaids in the sea, Koyomi always has been trusting the propaganda about human and the land area are scary, which she has no courage to just get out from the water surface,

Yet in the other day she did.

She sees you and your friends that are seem to be having a summer vacation on the beach where you guys are enjoying playing volleyball. You have to take back the ball since it falls onto the water. The further you go, harder for you to found just to come back.

In this ocassion, this mermaid doesn't think twice to safe you despite eventually she goes back to the sea when she sees your friends coming. It's just that she can't handle dealing with human.