Erill, whose original Japanese name is Eriru, is a character from the anime Mermaid Melody: Pichi Pichi Pitch. She serves as a secondary antagonist and is a member of the Dark Lovers, a group of water demons who are loyal minions to the main villain, Gackto. Her primary role in the story is to assist in capturing the mermaid princesses to help Gackto achieve his goals, and she works hard at this task even though she rarely succeeds.

In terms of physical appearance, Erill has brown hair that reaches her chest, which she typically styles into curled ringlets that frame her face. Her eyes are yellow, and she has fair skin with perpetual pink blush marks on her cheeks when she is in her normal state. She possesses several non-human features, including a pair of wings on her back, a visible fang, and cat-like ears that are usually covered by a yellow bandana with a leopard print pattern that she wears on her head. Her outfit as a water demon consists of a small dark blue tank top with thin straps, a matching short skirt with an orange-brown belt, and purple high heels. Like the other water demons, she has a human form that she uses to travel on land without attracting attention, though she can still use her attacks while in this form.

Erill's personality is complex and marked by a notable duality. Her usual demeanor is very cute, silly, and taunting, and in this state she is not particularly harmful or dangerous. However, she has a multiple personality disorder that manifests most clearly when she spins. When she spins, she transitions to a different personality; her face and voice change, and she screams in displeasure, particularly when hearing a mermaid's song. When she becomes angry or is fighting against the mermaid princesses, she shifts into her evil personality. In this state, her eyes glow red, her voice deepens, and her hands form claw-like shapes, making her much more menacing. There is also a third, more serious personality that appears less frequently, where her eyes look more normal and she becomes more observant of her surroundings. Despite her occasional arguments with the other Dark Lovers, she cares about them and considers them good friends. She is also deeply devoted to Gackto and tries her best to capture the mermaid princesses to make him happy. Among the Dark Lovers, she was considered the one most likely to win Gackto's affection if Sara had not found him first.

Erill possesses several notable abilities that she uses in combat and for espionage. In her water demon form, she attacks by firing black, wavy energy arrows at her opponents. She can also manipulate solid objects using her shadow-like powers and can create whirlwinds by spinning rapidly. Another ability is controlling fish to act as spies, allowing her to track the movements of the mermaid princesses. If she spins while on sand, she can dig down four meters. Her bandana serves a specific defensive purpose, as it is designed to protect her cat-like ears from the effects of a mermaid's song. However, this protection is limited; it is effective against a single mermaid's voice, but when two or more mermaids sing together, the bandana provides little protection, forcing her to rely on other methods like using sound systems to block out the music and harm the mermaids.