Hitagi Senjougahara is a third-year student at Naoetsu Private High School. Classmates initially perceive her as a frail, aloof "Lady in the Window" due to minimal social interaction and rumors of an incurable illness. She has long purple hair (depicted as dark brown in light novel artwork), blue eyes, and a slender frame, typically wearing her school uniform with black thigh-high stockings. Her appearance later evolves, including rolled-up sleeves and a short bob haircut after significant events.

Her background involves severe trauma. Surviving a childhood illness through extensive surgery led to her mother joining a destructive religious cult. Financial ruin and familial collapse followed when a cult member attempted to rape her; Hitagi defended herself by injuring the attacker with a spiked shoe. Her mother sided with the cult, abandoned the family, and left substantial debts. This trauma triggered an encounter with the supernatural "Heavy Stone Crab" (or "Weight Crab"), which took her emotional burdens and physical weight, leaving her at approximately 5 kilograms. To conceal her weightlessness, she carried heavy stationery items as ballast and used them as weapons when threatened.

Before this, Hitagi was an academically gifted and athletic middle school student, excelling in track and field and forming the close "Valhalla Combo" friendship with Suruga Kanbaru. Her personality shifted drastically after the trauma: she became guarded, verbally abrasive, and emotionally distant, self-identifying as a "tsundere" though others saw her as more "kuudere" (cold and reserved). She maintained a facade of superiority and rationality masking deep vulnerability and distrust.

Her development begins when Koyomi Araragi accidentally discovers her weightless secret after catching her during a fall on school stairs. Despite her initial hostility—including threatening him with stationery—Araragi insists on helping her. She accompanies him to seek aid from Meme Oshino, an expert in supernatural oddities. Through a ritual confronting her past, she regains her weight and emotional burdens, symbolizing acceptance of her trauma. This fosters trust in Araragi, leading to friendship and later romance.

As their relationship progresses, Hitagi's defenses gradually soften. She confesses her love during the "Mayoi Snail" arc, marking a pivotal shift toward emotional openness. She reconciles with Kanbaru and forms a complex bond with Tsubasa Hanekawa, even offering Hanekawa shelter after a house fire. Despite progress, possessive tendencies surface; in the "Karen Bee" arc, she attempts to shield Araragi from danger by confronting con man Kaiki Deishū—who previously exploited her family—but ultimately chooses to face Kaiki alongside Araragi, signaling growing trust and collaboration.

Her most significant growth occurs in "Hitagi End" (Koimonogatari), where she negotiates with Kaiki to save Araragi from Nadeko Sengoku. This forces her to abandon grudges and acknowledge Kaiki's role in her past without absolving him. She fully overcomes her trauma in "Tsukihi Phoenix," evolving into a more cheerful, ordinary teenager while retaining sharp wit. Post-graduation, she works internationally as a financial trader, maintaining a long-distance relationship with Araragi involving intermittent breakups and reconciliations, as depicted in "Musubimonogatari."

Throughout her journey, Hitagi demonstrates high intelligence, strategic thinking, and unwavering loyalty to those she loves, though her dependency on relationships remains a challenge. Her character arc concludes with a commitment to self-reliance and emotional authenticity, embodying the theme of carrying one's burdens while accepting support.