Tomari Kurusu shares a deep, lifelong bond with Hazumu Osaragi, forged in early childhood and enduring into high school. As inseparable toddlers, they accumulated countless shared memories. During their childhood, Tomari became Hazumu's protector against bullying stemming from Hazumu's feminine appearance and demeanor. She boldly vowed to protect Hazumu "forever," establishing a dynamic where Tomari embodied the adventurous, bold role and Hazumu the delicate, sensitive one. This dynamic persisted into adolescence. In one notable childhood incident, Hazumu proposed marriage, vowing to be Tomari's "bride." Tomari corrected Hazumu, stating boys are grooms, not brides, leading Hazumu to promise to become her groom someday.

Physically, Tomari possesses a tall, athletic build, standing slightly taller than Hazumu and being the tallest in their friend group. Her physique is thin, trim, and defined by musculature reflecting her athletic lifestyle. She has long legs contributing to agility and speed. Her facial features include a round face with prominent anime-style eyes featuring thick eyelashes. Her eyes have a reddish-purple hue that appears to shift shades with her emotional state. Her hair is deep amber with golden highlights, typically styled in thin twin tails secured close to her head. Her bust size is comparatively smaller than Hazumu's, a point of lamentation for her upon learning Hazumu's measurements. Tomari's standard attire is the female school uniform: a blouse with a bow, jumper dress, and knee-high socks. For athletics, she wears a polo shirt and buruma shorts. Casually, she favors tomboyish styles like t-shirts with shorts or capri pants, prioritizing comfort over fashion. She rarely wears dresses or denim skirts.

Tomari exhibits a gender fluid personality, blending traditionally masculine and feminine traits without conforming to typical female social roles. Her expression shifts between masculine and feminine depending on context and choice. While not transgender and showing no interest in transition, she falls under the broad transgender umbrella due to gender nonconformity. She is bisexual, attracted to males (including pre-transformation Hazumu and Asuta) and females (post-transformation Hazumu, Yasuna, and Ayuki). Dominant traits include strong leadership, proactive problem-solving, instinct-driven actions, and mediator capabilities in conflict resolution. She fearlessly confronts bullying or bigotry, consistently displaying protectiveness. Tomari deeply values friendship and loyalty, especially her bond with Hazumu, where the reversed protector-protected roles remain mutually acceptable. Socially comfortable with both genders, she sometimes struggles with intimacy, displaying bluntness or criticism that unintentionally hurts others. She maintains good physical and mental health through athletics and academics.

As an athletic and competitive student at Kashima High School, Tomari joined the track and field club. She excels in distance running, holding school records. Her masculine demeanor earns admiration from female teammates, with one explicitly stating she would develop strong feelings for Tomari if Tomari were male. Tomari avoids traditionally feminine activities like shopping, cosmetics, and fashion, exemplified by never wearing yukata until borrowing her mother's after Hazumu's transformation. Romantic feelings for Hazumu existed pre-transformation but intensified afterward, creating internal conflict as she struggled to acknowledge attraction to female Hazumu.

Following Hazumu's transformation, Tomari experienced significant turmoil. Ayuki observed Tomari preferred male Hazumu due to existing affections. Initially annoyed by others feminizing Hazumu, Tomari eventually accepted Hazumu's core personality remained unchanged. She attempted to shield Hazumu from distress caused by Yasuna's rejection, relenting when Hazumu expressed desire for Yasuna's friendship. Tomari's jealousy surfaced when witnessing or suspecting romantic moments between Hazumu and Yasuna, such as mistakenly believing she saw them kissing at a train station. This prompted Tomari to kiss Hazumu that night, asserting her own feelings. During a cultural festival, she interrupted another near-kiss, physically separating them. Her conflict peaked when Yasuna declared exclusive pursuit of Hazumu, leading Tomari to acknowledge her own romantic intentions.

The resolution of Tomari's romantic arc differs. In the manga, during a life-threatening fall, Tomari jumps to save Hazumu. Hazumu then confesses love, establishing their romantic relationship. The anime presents an alternate path: Hazumu initially chooses Yasuna to cure Yasuna's deteriorating vision, but they later separate. In the OVA, Yasuna organizes a Christmas event to unite Hazumu with Tomari. With help from friends and aliens Hitoshi and Jan Pu, Hazumu and Tomari enter a romantic relationship and eventually marry in a ceremony conducted by Hitoshi.

Tomari's key relationships include her deep, mutually dependent childhood friendship with Hazumu, characterized by reversed protector-protected roles. Her dynamic with Yasuna involves rivalry over Hazumu's affections, marked by tension and occasional scorn toward Yasuna's sensitivity. Interactions with Asuta feature bluntness and disciplinary actions he accepts without resentment. Ayuki maintains observational distance regarding the love triangle while providing insights into Tomari's feelings.