Koshino Natsuko is a classmate of Nagisa Misumi and Honoka Yukishiro at Verone Academy, sporting short hair that frames her face and dark brown eyes. She wears the school uniform or casual outfits, typically pairing a white long-sleeved top under a pale pink T-shirt with a dull moss vest featuring gold accents, goldenrod sneakers, and white socks.

Natsuko deeply admires Cure Black, finding her cool and cute, while viewing Cure White as lacking personality. Sharing common interests with Nagisa, her enthusiasm for Pretty Cure inspired her to create a detailed, handmade Cure Black costume for cosplay. This outfit includes a midnight blue cropped blouse with pearl pink lining and frilled sleeves, a white ribbon on the chest, light grey neck fabric, black shorts under a frilled skirt, fingerless gloves, a pale red belt with a heart center, and heart-shaped hair clips replacing Cure Black's usual accessories.

During one cosplay event with her friend Mori Kyoko, their realistic costumes led others to mistake them for the actual Pretty Cure heroes. This raised safety concerns from Nagisa and Honoka, the real Cures. Despite being ordinary cosplayers without powers, Natsuko and Kyoko were seen playfully acting as heroes for younger children. Although this inadvertently caused worry, Natsuko continued her cosplay activities afterward.