Eriru is a secondary antagonist in the Mermaid Melody: Pichi Pichi Pitch series, serving as a member of the Dark Lovers. This group consists of four water demons who are loyal followers of the series' primary antagonist, Gaito. As a water demon, her true form is that of a stingray, though she primarily appears in a human-like form to move through the world unnoticed and to engage in combat.

Eriru is most notably defined by a multiple personality disorder, which makes her behavior unpredictable and volatile. Her personality shifts are frequently triggered by spinning, an action she performs often. In her default, relaxed state, Eriru appears cute and somewhat silly, often engaging in playful activities like dancing or spinning simply for her own amusement. In this form, she is more of a nuisance than a serious threat, described as being talkative and not very harmful. However, when she becomes angry or is provoked during battle, her demeanor changes dramatically. She will spin rapidly, and upon doing so, her eyes glow red, her voice deepens and becomes darker, and her hands curl into claw-like shapes. This aggressive personality is cruel and powerful, focusing entirely on attacking her opponents. A third, rarer personality has also been noted, where she becomes more serious, observant, and capable of speaking normally, though this side of her is not seen often.

Her primary motivation stems from a deep devotion to Gaito. She works hard to capture the mermaid princesses in order to please him and earn his favor. Although she is a villain, she is not entirely without positive traits. Deep down, she cares for her fellow Dark Lovers, considering them good friends even though they often argue and compete for Gaito's attention. Her role in the story is to act as an obstacle for the protagonist mermaid princesses, using her unique powers and unstable nature in attempts to thwart their mission.

In terms of abilities, Eriru wields several powers tied to her water demon nature. She can launch attacks consisting of black, wavy energy arrows. Her spinning is not just a personality trigger but also a combat technique, as she can use it to create powerful whirlwinds. She is also capable of manipulating solid objects with shadow-like powers. A distinctive piece of her appearance is the yellow bandana she wears on her head. This accessory serves a defensive purpose, protecting her sensitive, cat-like ears from the harmful effects of a mermaid's song. However, this protection is limited; it is effective against the voice of a single mermaid, but she can be overwhelmed when multiple mermaids sing together. On such occasions, she has resorted to using sound systems to block out the music and counterattack.