Fuko Ibuki is a first-year student at Hikarizaka Private High School, though this status is complicated by her unique circumstances. She is the younger sister of Kouko Ibuki, a former art teacher at the school. Shortly after her high school entrance ceremony, Fuko was involved in a severe car accident that left her in a comatose state, forcing her to remain hospitalized for years. During this time, a spiritual manifestation of Fuko, driven by her powerful desire to do something meaningful for her sister, appeared at the school. This "ghost" Fuko took up residence in an old, empty classroom, dedicating herself to carving small wooden starfish.

In terms of personality, Fuko is characterized by a profound childlike innocence and eccentricity. She is a natural airhead, often drifting into a fantasy world, especially whenever she sees or thinks about starfish. During these "starfish daydreams," she becomes completely oblivious to her surroundings, a quirk that others often take advantage of for lighthearted teasing. Despite her small stature and immature behavior, she firmly believes she possesses an adult-like demeanor and becomes irritated when treated like a child. She speaks in a very direct and honest manner, frequently referring to herself in the third person by her own name, "Fuko." While she can be a bit rude and stubborn, particularly towards the protagonist Tomoya Okazaki and his friend Youhei Sunohara, this is often a front for her underlying shyness and past difficulty in making friends.

Fuko's central motivation throughout the narrative is her love for her sister, Kouko. Her sole mission is to ensure that as many people as possible attend Kouko's wedding to Yusuke Yoshino. To achieve this, her spirit tirelessly carves hundreds of star-shaped wooden figures, which she gives away as invitations. Her hope is to fill the wedding with well-wishers, providing her sister with happiness even while her real body lies unconscious in a hospital bed.

Her role in the story is initially that of a mysterious "ghost" whom Tomoya and Nagisa Furukawa encounter. They become her primary supporters, helping her carve and distribute the starfish to the entire school, a task that proves difficult as Kouko is no longer a teacher known to most students. As Fuko's mission progresses, a heartbreaking phenomenon occurs: as her physical condition in the hospital worsens, people begin to forget about her spiritual existence. She fades from the memories of those she met, including Tomoya and Nagisa, leading to a poignant climax at the wedding ceremony itself. After the wedding's success, her spirit fades away, but the story of a girl handing out starfish leaves a lingering impression on everyone.

Key relationships define her journey. Her bond with her sister Kouko is the emotional core of her character, representing pure, selfless love. With Tomoya and Nagisa, she forms a deep friendship; they are the first to truly help her, and despite Tomoya's constant teasing, she grows to cherish them. Tomoya often calls her a "dangerous carving girl" due to the numerous bandages on her hands from carving accidents. This friendship also introduces her to Nagisa's family, the Furukawas, who warmly welcome her into their home. Her persistence and childlike charm even inspire a group of schoolboys to form an unofficial "Fuko Bodyguard Squad" to protect and defend her honor.

Fuko undergoes significant development primarily in the "After Story" arc. Through a miracle, represented by a "Light Orb" from the town, Fuko eventually awakens from her coma. However, having been asleep for roughly seven years, she has continued to age chronologically but has not matured mentally, retaining the exact same childlike personality she had at fifteen. This leads to comedic situations where she, a young woman, is often seen as a peer or even an older figure to Tomoya and Nagisa's young daughter, Ushio Okazaki. Fuko becomes intensely attached to Ushio, wanting to be her older sister and frequently trying to "take her away" from Tomoya, against whom she continues her playful rivalry. They become close playmates, and Fuko's presence brings a sense of lingering connection to the past's miracles.

While her most prominent ability is the supernatural projection of her spirit, her notable everyday skill is her dedication to carving. She is exceptionally focused and persistent in her craft, a trait that is both endearing and slightly alarming to those around her. Her most defining characteristic, however, is not a superhuman talent but her unwavering determination to act on her love for her sister, a simple and powerful will that ultimately bridges the gap between life and the spirit world.