Sumire Miyazaki is a minor character in Happy Sugar Life. She is Satō Matsuzaka's next-door neighbor and co-worker who refers to her as "senpai." Among her friends, her nickname is Su-chan. Sumire is a self-conscious girl, referring to herself as plain and awkward and idolizing Satou for being the exact opposite of herself.

Spoiler

She has a rather obsessive personality, becoming infatuated with Satou, blushing over sniffing her work uniform, and going as far as to break into her locker to know more about her.

(Source: Happy Sugar Life Wiki)