Natsumi Hayamiya is Tsutomu Senkawa's childhood friend and classmate, intimately familiar with his habits since their early years—particularly his penchant for exploring abandoned buildings. She dismisses this hobby as childish yet consistently joins him during impromptu encounters, though she later loses all memory of ensuing alien-related incidents.

At school, she chairs the journalism club under Keisuke Muroto's mentorship, who identifies her untapped potential. She cultivates friendships with classmates, including the habitually reserved Sayaka Nakasugi. Her bold, occasionally rash pursuits as a student journalist starkly contrast her profound loyalty and protective instincts toward friends. Despite jibes from peers like Ryouta Sudou—who mocks her romantic prospects due to her temperament—she radiates unshakable self-confidence, brushing off such remarks without hesitation.

During the Ryunka incident investigation, she accompanies Muroto to Sawajiri Village in Nagano Prefecture, witnessing firsthand the chaotic disturbances linked to the event. Birdy intervenes to shield them from armed operatives employed by antagonist Satyajit Shyamalan. Natsumi also participates in casual gatherings with Tsutomu and classmates, such as Sayaka Nakasugi's welcome-back celebration at her family mansion.