Muromi-san is the titular mermaid of the series Namiuchigiwa no Muromi-san. She physically appears as a cheerful teenage girl with green hair styled into twintails, violet eyes, and two clam shells adorning her head. In reality, she is an ancient legendary creature who has lived for thousands of years, and along with her sisters, serves as a guardian of the Earth. She speaks in the Hakata dialect and frequently comes ashore to enjoy human culture, showing a particular fondness for sake and sweets. Her personality is energetic, talkative, impulsive, and reckless, often acting without considering the consequences of her actions. She can be dense and prone to violent outbursts, yet she also displays a genuine, if eccentric, kindness toward those she considers friends. Her motivations are largely driven by a desire for fun, social interaction, and attention, especially from the human boy Takurou Mukoujima, whom she affectionately calls Takkun. She latches onto him after he accidentally catches her while fishing, and she continually seeks his company even though he is mostly uninterested and just wants to fish in peace. This sets the stage for her role in the story as the primary catalyst for the series' comedy and chaos, dragging Takurou into adventures involving a variety of mythical sea creatures and legendary beings she invites to the shore. Key relationships include her mermaid friends Levia-san, Hii-chan, and Sumida-san, who often try to help her or rein in her more irrational behavior. She also interacts with other legendary creatures such as Yeti, Harpy, and Fuji-san. A notable aspect of her backstory is her intense hatred of marine mammals like whales and dolphins, stemming from a belief that they are descendants of a boyfriend she had during the Mesozoic era who developed lungs but chose to stay in the ocean. This grudge is a recurring point of humor and insight into her long, storied past. As a character, Muromi-san does not undergo significant development across the series, instead remaining a consistent source of high-energy absurdity. Her notable abilities include being physically the strongest among mermaids, consistently winning mermaid competitions, and possessing the general traits of a mythological ningyo, allowing her to live for millennia and survive both on land and in the sea. Her true nature as an ancient guardian hints at powers beyond her playful demeanor, though the series typically focuses on her comedic antics rather than her supernatural duties.