Kaguya Shinomiya is one of the main characters of Kaguya-sama: Love is War and serves as the vice president of the student council at Shuchiin Academy, an elite and prestigious high school. She is the daughter of the Shinomiya family, the head of one of Japan's largest and most powerful business conglomerates, the Shinomiya Group, which boasts total assets of 200 trillion yen. However, her position within this family is complicated, as it is revealed that Kaguya is an illegitimate child born from her father's affair with a woman named Nayotake Shimizu, who died shortly after giving birth to her. Consequently, she is treated more as a political tool than a true family member by her older half-brothers, despite her father still recognizing her.

Kaguya is a versatile genius who excels in academics, the arts, and music. Her talents extend to martial arts, as she holds a first-degree black belt in Aikido and a second-degree black belt in Judo, and she is also a member of the archery club with skills at the national competition level. Despite this immense intelligence and capability, she has notable weaknesses. Due to her sheltered upbringing, she is surprisingly naive about the common world, struggling with modern technology and social media. She is also very inexperienced in matters of romance and sexuality, possessing a very low tolerance for dirty jokes.

Her personality is a carefully crafted duality. On the surface, she presents herself as a proud, cold, and calculating individual, a demeanor that earned her the nickname Ice Princess Kaguya during her middle school years. This behavior stems from a strict upbringing that isolated her and taught her to view others in terms of their utility, leaving her with a deep-seated loneliness and an inability to easily understand the feelings of others. Beneath this exterior, however, she is revealed to be an innocent, kind, and fair-minded girl who genuinely cares for her friends. This inner conflict manifests as internal debates between different facets of her psyche, such as the logical Kaguya (Ice) and the emotionally suppressed Kaguya (Fool).

Kaguya's primary motivation throughout much of the story is her love for the student council president, Miyuki Shirogane, for whom she harbors deep feelings. However, her overwhelming pride, a product of her family name, prevents her from confessing. She believes that whoever confesses their love first will be the loser in the relationship. This leads to a series of elaborate psychological battles and mind games, as she constantly devises schemes to manipulate Miyuki into confessing his feelings to her first, while stubbornly refusing to acknowledge her own actions as love.

Her role in the story evolves through her key relationships. Her bond with her personal valet, Ai Hayasaka, is exceptionally close; they have been together since childhood, sharing a sisterly bond where only Kaguya understands Hayasaka's flaws and only Hayasaka fully understands Kaguya's. Her friendship with the student council's secretary, Chika Fujiwara, is more complex. While Chika is often a source of frustration and jealousy for Kaguya, as she easily attains things Kaguya cannot, Kaguya ultimately considers their friendship to be as irreplaceable and as important as her feelings for Miyuki. She also acts as a caring senior to her underclassmen, Yu Ishigami and Miko Iino, showing concern for Ishigami's well-being and considering Iino a good person.

Throughout the series, Kaguya undergoes significant character development. Her time in the student council, and her interactions with Miyuki and her friends, gradually helps her shed her cold and cynical mindset. She begins to believe in genuine kindness and becomes more socially adept. This development culminates in the mutual confession of feelings between her and Miyuki, after which their relationship deepens, and she eventually supports his decision to study abroad.