"Fiancée Shitai" centers on the life of Shougo, a high school student who unexpectedly becomes the guardian of his younger half-sister, Rui, after their parents pass away. Shougo, who had been living a carefree life, is suddenly thrust into the responsibilities of adulthood, balancing school, work, and caring for Rui. The story explores their evolving relationship as they navigate the challenges of their new reality.

Rui, initially reserved and distant due to the trauma of losing her parents, gradually opens up to Shougo, relying on him for emotional support. Shougo, in turn, struggles with his newfound role as a guardian, often feeling overwhelmed but determined to provide stability for Rui. Their bond deepens as they face everyday struggles, from financial difficulties to societal judgments about their unconventional living situation.

The narrative also introduces other characters who influence their lives, such as Shougo’s classmates and coworkers, who provide both support and complications. Romantic undertones emerge as Shougo and Rui’s relationship becomes more complex, blurring the lines between familial love and romantic affection. The story delicately addresses themes of grief, responsibility, and the boundaries of relationships, presenting a nuanced portrayal of two individuals rebuilding their lives together.

As the series progresses, Shougo and Rui confront external pressures and internal conflicts, forcing them to reevaluate their feelings and the nature of their bond. The manga balances moments of heartfelt emotion with lighthearted interactions, creating a narrative that is both poignant and relatable. Through its exploration of family dynamics and personal growth, "Fiancée Shitai" offers a compelling and thought-provoking story.