Haya Magari is the older sister of the protagonist Isaki Magari in the anime Insomniacs After School. She is four years older than Isaki and is a university student attending a school in Kanazawa City, the prefectural capital. Her personality is flamboyant and she behaves in a commanding, queen-like manner towards her younger sister, who has described her as a gal who debuted in college. Haya is a playful and unapologetic prankster, known for wearing her sister’s clothes and even drawing on Isaki’s face while she is asleep.

Haya’s motivations and complex relationship with Isaki stem from their childhood. Due to Isaki’s frail health and a heart condition that has made her the consistent focus of their family’s attention and worry, Haya grew up feeling neglected and resentful. However, this resentment is not born of pure malice. A pivotal memory for Haya was watching a young Isaki struggle in a race. While others saw a fragile child to be helped and pitied, Haya witnessed Isaki crying into a tree afterwards, expressing her own frustration at being treated differently. From that moment, Haya decided her unique way of showing affection would be to never treat her sister as special or an object of pity. Her harsh teasing and refusal to coddle Isaki are therefore her way of offering a normal, untempered relationship that she believes her sister truly desires.

In the story, Haya serves as a critical bridge and evaluator in the developing relationship between Isaki and Ganta Nakami. She appears in person when Kurashiki-sensei visits the Magari home to get permission for the astronomy club's training camp. Haya joins the trip to their grandmother’s vacant seaside house, acting as both an agent of chaos and a watchful chaperone. While her antics provide comic relief, they also serve a deeper purpose. She deliberately delays Isaki to have a private conversation with Ganta, directly questioning his intentions and warning him not to get involved with her sister out of pity. When Ganta firmly and sincerely replies that he has never once felt sorry for her, and instead sees her as someone who shines brightly, Haya’s demeanor shifts. Her true, caring nature becomes visible as she shares the story from their past, revealing the reason for her behavior. In a symbolic gesture, she has Ganta give her a high-five, essentially tagging him out and entrusting her sister into his care before leaving the two alone for the remainder of the camp.

Throughout the series, Haya demonstrates a subtle but significant development, transforming from a seemingly mean older sister into a nuanced and protective figure. Her key relationship is, of course, with Isaki, characterized by a tough love that masks deep-seated care. Her relationship with Ganta evolves from suspicion and testing to trust and approval as she recognizes his genuine feelings. While Haya possesses no special abilities, she is notable for her perceptiveness, using her chaotic energy to skillfully gauge Ganta’s character, and her practical helpfulness, as when she drives them to the camp.