Sae Kusunoki possesses brown hair featuring spiky bangs, straight waist-length locks, and an ahoge, paired with green eyes. Her physical build is that of a slim, pale teenager with medium breasts. She appears in diverse attire, including a school uniform, a maid's dress with headdress, thigh-high stockings, pajamas, an old-type school swimsuit with swim cap, a mini-dress, a collar, a choker, exposed clothing, and a gauntlet.

An ordinary Japanese schoolgirl without notable academic or athletic prowess, her selection as a magical girl initially causes confusion. She was chosen due to her high latent magical potential, necessary to fight interdimensional threats. A fairy named Emito serves as her constant guide and companion after identifying this potential.

Personality-wise, Sae exhibits clumsiness, timidity, and self-doubt about her suitability for the role. A significant aspect of her magical transformation involves temporary nudity, causing her acute embarrassment and reluctance to recite the required incantation "Twinkle Starlight – Warm up!". Her magical abilities stem directly from her willpower and imagination, allowing her to conjure energy projectiles or manifest weapons. However, she gravitates toward summoning heavy artillery in combat, a tendency that clashes with Emito's preference for subtler methods.

Throughout her documented experiences, she confronts evil magicians from the parallel dimension Eetel Land, who harvest human consciousness for power. She faces adversaries like Ruran and later Baldoi, who attacks her alongside influenced male students. A development occurs when her acquaintance Shouko demonstrates the ability to perceive Emito, revealing Shouko's own latent magical potential. Sae expresses reservations about Shouko becoming a magical girl, citing the traumatic nature of her own encounters.