"Happiness Girl" centers on the life of Aoi Fujino, a high school student who appears to lead an ordinary life but harbors a unique ability: she can see and communicate with spirits. This ability, inherited from her grandmother, often complicates her daily life as she navigates the challenges of adolescence while assisting restless spirits in finding peace. The narrative explores themes of empathy, responsibility, and the coexistence of the living and the dead.

Aoi’s encounters with spirits range from heartwarming to harrowing, each presenting a distinct story that intertwines with her personal growth. Her interactions with these spirits often reveal unresolved emotions or unfinished business tied to their past lives, and Aoi’s role is to help them achieve closure. Alongside her is her childhood friend, Haruto Soma, who is initially skeptical of her abilities but gradually becomes a supportive ally. Haruto’s grounded personality provides a counterbalance to Aoi’s more emotional and intuitive nature.

The series also introduces a mysterious spirit named Rei, who becomes a recurring figure in Aoi’s life. Unlike other spirits, Rei retains a significant portion of his memories and exhibits a deeper connection to the spiritual realm. His presence raises questions about the boundaries between the living and the dead, as well as the nature of his relationship with Aoi. Rei’s enigmatic past and his growing bond with Aoi add layers of intrigue to the story.

As the narrative progresses, Aoi’s abilities attract the attention of both benevolent and malevolent forces within the spiritual world. She faces challenges that test her resolve, including encounters with vengeful spirits and entities that seek to exploit her powers. These conflicts force her to confront her own fears and limitations while deepening her understanding of the spiritual realm.

The manga balances episodic storytelling with an overarching plot, weaving together individual spirit encounters with Aoi’s personal journey. Themes of loss, forgiveness, and the enduring impact of human connections are recurrent, as each spirit’s story reflects universal emotions and experiences. The series maintains a focus on Aoi’s development as she learns to navigate her dual role as a high school student and a mediator between worlds, all while uncovering the mysteries surrounding her abilities and the spiritual realm.