Merielle Sim Omui is the daughter of Lord Omui, the feudal lord of the border city of Omui. She is known among the locals as the Twin-Sword Princess, a title that reflects her skill with dual blades. Despite this formidable reputation, her true personality is quite different: she is secretly a tearful and scatterbrained young woman who struggles to maintain her composed public image. She deeply respects her father and aspires to emulate him, though her own emotional vulnerability often undermines her efforts.

Her role in the story begins when she is attacked by thieves while traveling in a carriage. She is rescued by the protagonist Haruka, and from that point onward she becomes a recurring supporting character in his adventures. She frequently finds herself flustered and frustrated by Haruka's habit of calling her Merimeri or Merimeeri-san instead of her proper name, a recurring gag that highlights her inability to control the situation around him. Her relationship with Haruka is defined by a mix of gratitude, exasperation, and ongoing attempts to assert her dignity, none of which succeed.

Merielle's development is relatively modest within the narrative. She starts as a dutiful noble daughter who tries to uphold her position, but her encounters with Haruka gradually expose her clumsy, emotional side. She is prone to crying and being scatterbrained, yet she remains genuinely well-meaning and devoted to her duties. Her notable ability lies in her twin-sword combat style, a practical skill befitting the daughter of a border lord who must defend his territory. Beyond that, she is characterized more by her personal quirks than by any extraordinary powers or magical talents. Her physical appearance includes blonde hair, green eyes, and a height of 169 centimeters.