Windom, operating under the human alias Reika Shirogane, is a 16-year-old female with striking silver hair, golden eyes, and distinctive glasses. Her family name "Shirogane" meaning "silver" in Japanese, subtly nods to the metallic nature of her home, Planet Metal. She favors her old school uniform for casual wear.

Reika projects a serious, intellectually curious persona, yet privately nurtures a passionate obsession with Boys' Love manga, especially the series "ScoreYou." Deeply embarrassed by this interest, she conceals it from friends, even inventing a cover story about her first transformation happening while studying educational texts—a fabrication masking the truth that it occurred during a BL reading session. This hidden passion sometimes betrays her, like when mentions of "ScoreYou" trigger visible blushes.

As a GIRLS member, Reika carries the soul of the kaiju Windom, once a capsule monster of Ultraseven. Discovering her kaiju soul led her to enroll in GIRLS, where she quickly bonded with fellow newcomers Aki Miyashita (Agira) and Miku Ushimaru (Miclas), forming the core "Capsule Girls" trio. While central in the first season, her role alongside Miku's diminished in the second.

She accesses her kaiju abilities via a Soulrizer device; attempting transformation without it risks a berserk state. In her kaiju girl form, she employs goggles derived from Windom's eyes as sniper scopes, firing long-range laser beams from her forehead components during battles against shadow entities. She also competes in GIRLS' "Big Kaiju Battles" sporting events.

Reika appears throughout both anime seasons and the film "Kaiju Girls (Black)." Her design originates from the Ultra Kaiju Anthropomorphic Project, specifically utilizing ASCII Media Works' interpretations.