Melu is a mermaid princess and a member of the main household in the series. Her full name is Meroune Lorelei de Neptune, and she is the legitimate royal heir to the merfolk kingdom, often going by the nickname Mero. As a mermaid, her upper body is that of a graceful young woman with long arms and webbed hands, while her lower half is a large, elegant fish tail complete with fins. To move around on land, she relies on a wheelchair, which she eventually upgrades to a motorized version. Her body secretes a layer of mucus to keep her skin from drying out, and she requires a pool in her room for comfort.

In terms of personality, Melu is gentle, polite, and refined, often serving as a calming presence and a voice of reason among the more boisterous monster girls in the house. However, her most defining trait is her deep and abiding love for tragic romance. She is obsessed with sad love stories and has a powerful tendency to get lost in dramatic and melancholic fantasies about love, often imagining herself in the role of a heartbroken lover or a tragic mistress. This fascination stems from the famous, sorrowful tale of a mermaid princess from her culture. Despite her noble and composed demeanor, she can be surprisingly mischievous and athletic, possessing significant strength and agility, especially in the water.

Melu is the fifth non-human girl to join Kimihito Kurusu's household, and her arrival was intentionally orchestrated by the exchange coordinator Kuroko Smith without prior notice. Her primary motivation at first is not to become a bride, but to experience a beautiful, tragic, and unrequited love. She idealizes the role of a mistress whose love is doomed, and she initially tries to cast herself in that position in relation to Kimihito. However, as she spends time with him and the rest of the household, her feelings evolve. She moves away from her fantasy of tragic love and develops genuine, earnest affection for him, eventually declaring her wish to be his number one. She develops a close relationship with Kimihito, whom she addresses as Master, and acts as a peacemaker and mediator in conflicts among the other girls. A key relationship is with her mother, the queen of the merfolk, who is strict and protective, forbidding Melu from working a job and creating a source of playful frustration for the princess.

Melu undergoes a significant development in her character arc. Her initial worldview, which romanticizes heartbreak and melancholy, is challenged and reshaped by the genuine and happy connections she forms in the Kurusu household. She reconciles her fairy-tale ideals with real-world affection, learning to openly value a loving relationship over a tragic one. Her noble status also plays a role in the story, as she is recognized in public and uses her allowance from her family to help with household expenses.

Melu possesses several notable abilities tied to her aquatic nature. She is an astonishingly fast swimmer and can leap high out of the water with ease. On land, she is far from helpless, as she can use her powerful tail to deliver a strong blow, enough to knock over other energetic monster girls. Her skin's protective mucus also provides resilience against cold conditions. While her dress is designed to stay in place despite her slippery skin, it is prone to occasional comedic mishaps.