Hijiri Yajima is a central character in FLCL Alternative, where she is introduced as one of the closest friends of the protagonist, Kana Koumoto. She is a high school student who carries herself with an air of confidence and maturity that sets her apart from her peers. Physically, Hijiri is depicted as a tall and striking beauty, characterized by her long, voluminous black hair with wispy bangs, piercing blue eyes, and a soft jawline with a sharp chin. Her fashion sense is notably stylish, whether she is dressed in her school uniform, which she wears with a sense of personal flair, or in a variety of sophisticated casual outfits.

Initially, Hijiri's personality appears to be the most mature among her group of friends. She has found success as a model, having appeared on a magazine cover, and approaches this with a coy sense of sarcasm. This image of maturity is reinforced by her romantic life; she turns her attention away from boys her own age and begins dating an older college student named Toshio Shioya, a relationship that her friends greatly admire. This persona, however, is a carefully constructed facade. As the story unfolds, it becomes clear that Hijiri has been pretending to be more grown-up than she actually is. This charade is challenged after her breakup with Toshio, which prompts a significant shift in her character. She stops modeling, takes up a job as a waitress at a karaoke restaurant, and begins to embrace a life more typical of a teenager. She openly admits that her previous "mature" behavior was inauthentic and learns to enjoy her more "immature" side alongside her friends.

Hijiri's role in the story is intrinsically linked to her friendship with Kana, who looks up to her as the pinnacle of adulthood. Kana's own immaturity leads her to become deeply invested in Hijiri's seemingly sophisticated relationship, to the point of trying to "fix" it behind her back. This intervention leads to a confrontation that serves as a crucial learning experience for both characters. While Kana is forced to confront her own unwelcome actions, Hijiri redirects her discontent with the situation toward Toshio, recognizing that his own careless indecisiveness mirrors the false persona she had been projecting. This moment of self-realization helps Hijiri understand that she lost sight of what is truly important, ultimately strengthening her genuine friendships. Throughout the series, her development is marked by this transition from an aloof and "mature" figure to a more grounded and authentic person who values her friends over appearances.

A notable moment that highlights her growth occurs when a falling light post endangers Toshio. Despite their breakup, Hijiri rushes in and saves him, demonstrating a core empathy and decisiveness that her earlier facade often masked. When Toshio later tries to reconcile, claiming to like her more, Hijiri stops her friends from defending her and explains that she and Toshio are alike in their pursuit of a false maturity. In that moment, she rejects the return to her old persona, solidifying her commitment to being her true self.