Haku Nishidate is a first-year middle school student and member of the idol unit Tetrarkhia. She possesses fair skin, sharp deep blue eyes, and distinctive white hair styled in an angled cut featuring a bun on the left side of her head and spiked bangs at eye length, complemented by near-chest-length forelocks layered over them. Her casual style favors cutesy designs with cool accents, while her Tetrarkhia performance outfit includes ripped grey tights.

Personality-wise, Haku is a quiet, lone-wolf type who typically avoids group activities. Despite this inclination, she follows Mikuru Bandou's lead and contributes significantly to Tetrarkhia, enabling the unit's group activities to materialize. She enjoys mint chocolate and cats, occasionally skipping practice to play with them. She dislikes ball games and maintains a stoic demeanor. Contradictorily, she considers herself Tetrarkhia's leader, a self-perception shared by all four unit members.

Her background reveals no extensive personal history, focusing on her participation in Tetrarkhia's activities and performances. The unit, formed in Aichi Prefecture, gained recognition during the Prism Stage Preliminaries in the Chūbu region. Her surname, "Nishidate," incorporates the character for "west" (西), aligning with Tetrarkhia's naming convention using cardinal directions and symbolically linking her to the White Tiger of the West in Chinese mythology.

Haku appears in the anime during reconnaissance activities alongside KiRaRe at Marehoshi Academy's main campus and performs in the Prism Stage competition.