Trapped within a small glass bottle sealed by a metal lid and chain, Merc exists as a mysterious entity. She awakens from a liquid state into a humanoid form, bearing no memory of her life before the bottle. Her body is composed entirely of water, granting her light skin, blue eyes, and hair that flows like liquid. She wears a simple white sleeveless dress accented by a violet ribbon. Outside her container, her lower body dissolves back into water, and anger summons horns upon her head. The bottle itself holds sunken rings and pearls at its base and floating pink flowers within its water. While usually requiring carriage, she occasionally floats independently. Her watery nature often leads observers to mistake her for sentient water, a monster, or a fairy, and it prevents her from consuming food.

Despite her confinement, Merc radiates a cheerful, expressive, and sociable nature, forming a stark contrast to her timid companion, Yuu. She is polite and makes friends easily. Her interests lie in reading romance stories—sometimes losing herself completely within them—and collecting rings and other cute objects. She reacts with distinct irritation if called merely "water," insisting on her name. The void of her forgotten past fuels her primary drive: a journey to discover her origins.

Her known history begins when Yuu's father purchases her sealed bottle from a peddler for 10 gold coins. Upon awakening, she embarks on a quest with Yuu across the fantasy world of Mehterhane to reclaim her memories. Their travels involve healing monsters' hearts to resolve conflicts and aiding diverse communities, adapting several Country Event stories. The journey concludes without resolving her amnesia or altering her watery form, leaving her core quest for self-discovery ongoing. Key relationships center on her dynamic with Yuu, whom she actively encourages and supports, alongside fellow travelers like Toto and Djamo.