"Marshmallow Fiancée" is a romantic comedy manga that centers on the relationship between two individuals brought together by an unconventional arrangement. The protagonist, a young man named Aoi, is a reserved and hardworking individual who struggles with social interactions. His life takes an unexpected turn when he encounters a mysterious girl named Mashiro, who claims to be his fiancée. Mashiro is sweet, cheerful, and somewhat naive, with a childlike demeanor that contrasts sharply with Aoi's serious personality.

The story unfolds as Aoi learns that Mashiro is not an ordinary girl but a "marshmallow spirit," a supernatural being who has chosen him as her partner. Despite his initial reluctance, Aoi gradually becomes drawn to Mashiro's warmth and sincerity. Their relationship develops through a series of comedic and heartfelt moments, as Mashiro’s presence challenges Aoi to confront his insecurities and open up emotionally. The narrative explores themes of love, trust, and personal growth, as both characters navigate the complexities of their bond.

Supporting characters add depth to the story, including Aoi’s friends and family, who react with varying degrees of skepticism and support to his unusual situation. Mashiro’s origins and the rules governing her existence as a marshmallow spirit are gradually revealed, adding layers of intrigue to the plot. The manga balances lighthearted humor with tender moments, creating a dynamic interplay between the fantastical elements and the grounded emotional core of the story. Through its blend of romance, comedy, and supernatural elements, "Marshmallow Fiancée" offers a charming and engaging narrative that appeals to fans of the genre.