Kii Saegusa is introduced as a main character in the second season of the Aikatsu! anime, representing Dream Academy as a student who balances the roles of both an energetic idol and a meticulous producer. She hails from an island where her mother works as a doctor, and her mother has been a significant source of support and confidence throughout her life. A formative childhood experience involved intense shyness and social isolation, driven by envy of peers she perceived as cuter. Her mother intervened by casting a symbolic confidence-boosting "spell," which proved foundational and motivated Kii to eventually enroll at Dream Academy.

In terms of appearance, Kii has bright blonde hair styled into puffy, balloon-like twintails, each secured with a green ribbon. She has cerulean eyes and, when not performing on stage, is typically seen wearing a pair of red glasses. Her personality radiates energy and cheerfulness, which aligns with her designation as a pop-type idol. This exuberance is marked by a distinctive speech pattern where she repeats certain words two or even three times before ending a phrase or sentence, a trait she revealed to have inherited from her mother and is exemplified by her catchphrase "Ok-ok-okay!". Her preferred brand for performances is Magical Toy, which reflects her pop-type image.

Kii's unique significance in the story comes from her dual role. She is not only a skilled idol but also a dedicated producer, most notably for her close friend and fellow Dream Academy student, Seira Otoshiro. She takes her job as a producer seriously, leveraging sharp analytical skills and extensive research. She uses a computer system nicknamed "compy" to gather information and strategize performances, blending data-driven plans with creative flexibility to support her friends and optimize their stage appearances. Initially, her focus was solely on producing, but she gradually overcame her hesitancy to embrace her own idol career, learning to successfully balance both roles. She is also known for helping Seira with her debut and is considered a "great producer" entrusted with guiding her career. Kii's relationship with Seira is a central part of her story, characterized by deep affection, support, and a strong professional partnership. Her relationship with her mother, whom she also refers to as "her producer," remains a key influence.

Throughout the series, Kii achieves several notable milestones. She is recognized as the first idol to perform the Pop Flash special appeal. She also obtains her first Premium Rare Coord, the Unicorn Sagittarius Coord, in Episode 66. She is known to have attended the Aikatsu! Boot Camp for training. In terms of character-specific distinctions, Kii remains the only main character from her generation who never joined an official unit, and she is the only second-season protagonist who never performed the song SHINING LINE*. Among her personal preferences, her favorite beverage is Brain Thunder, her favorite food is Pon Pon Crepes, and her least favorite is spicy curry. Her performance aura consists of a single toy alicorn wearing a pearl necklace, accompanied by bubbles filled with whimsical objects.