Yotsuyu Hirasaka is a 15-year-old high school student whose name translates to "night dew" (夜露). Her surname, Hirasaka, combines the meanings "ratio, proportion" (比), "good, virtuous, respectable" (良), and "slope, hill" (坂). She possesses distinctive pink hair and lilac eyes, stands 1.56 meters tall, and has blood type AB. Within the Alice Gear Aegis narrative, Yotsuyu serves as an Actress for Narikozaka Manufacturing, piloting specialized combat suits called Alice Gears to protect humanity from extraterrestrial mechanical threats known as Vice. Her mere presence triggers an extreme reaction in Nodoka Takahata, causing intense blushing, nosebleeds, and fainting spells that form a recurring comedic element.

A significant part of Yotsuyu's background includes her past leadership of the Crimson Glory biker gang. She successfully persuaded the group to abandon fighting and transition into community service activities, showcasing her leadership and reformist capabilities. Her story unfolds across multiple franchise installments. She appears in the original video animation "Alice Gear Aegis: Doki! Actress Darake no Mermaid Grand Prix" and continues her role in the 2023 television anime series "Alice Gear Aegis Expansion" (airing April-June). In the series, she participates in various group endeavors with fellow Narikozaka Actresses, including festival preparations, survival games, operating a concept cafe, a Mount Takao hiking trip, and community service initiatives. These activities occur against a broader sci-fi backdrop.