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 to the point that, in Lucia's perspective, he has forgotten his mermaid love completely, and is desperate that he return her feelings. 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).
In the manga, when Kaito states that he loves only Lucia and could never love Mikaru the way that she loves him, Mikaru 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. In the end, Mikaru is reborn and given to her brother to raise her from scratch.
