Mikaru is the girl who finds Kaito after he had lost his memories. She's a rich, yet desperately lonely girl who has lost all hope. She falls in love with Kaito and is desperate for him to return her feelings to the point that, from Lucia's perspective, he has forgotten his mermaid love completely. Mikaru also seems to be a sickly girl, collapsing to the floor every time Mikeru appears somewhere else or when Kaito is near Lucia (in order to keep Kaito to herself).

Spoiler

In the manga, when Kaito states that he loves only Lucia and could never love Mikaru the way that she loves him, Mikaru takes off her clothes and offers herself to him in desperation of keeping him; Kaito turns away from her and leaves. This is a large trigger for her collapses and allows Mikeru to take control of her.

Spoiler

In the end, Mikaru is reborn and given to her brother to raise her.