Hayu Nukui (貫井 はゆ) is a 15-year-old high school student and core member of the idol group Fruit Tart. Hailing from a wealthy Kanagawa family, she ran away after her father opposed her music career aspirations, leading her to independent living at Nezumi House while signed to Rat Productions.

Her appearance features light blonde hair, cyan eyes, and a distinctive snaggletooth. She wears a flat white hat with a black ribbon, multiple black hair clips, a red patterned jacket layered over a white button-up shirt, a navy blue and white frilled skirt, black pantyhose, and tan boots.

Charismatic and tomboyish with a rock-inspired persona, Hayu secretly nurtures a feminine side—indulging in girls' comics and harboring a childhood dream of becoming a bride. This duality originates from her strained relationship with her overbearing mother and disapproving father. Despite her wholesome, innocent nature within the group, she cannot sing; instead, she contributes as Fruit Tart’s guitarist and songwriter, collaborating closely with classmate and fellow member Ino Sakura.

Past loneliness fuels Hayu’s profound gratitude for her friendship with Ino, though she grows sensitive to Ino’s rising popularity, fearing fan attention might weaken their bond. Shyness surfaces in her physical reactions, such as blushing when hugged by Ino or receiving compliments. She initiated their school friendship and often uses hugs to encourage or comfort Ino. Her surname "Nukui" combines the characters for "pierce/penetrate" (貫) and "well/mine shaft" (井).