Flandre is a gynoid constructed by Professor Frankenstein to serve the Monster Realm's royal family, specifically assigned as Princess Hime's bodyguard and maid. She possesses a child-like physical appearance characterized by light brown or yellow hair styled in a bun secured by a bolt, droopy purple eyes, and a perpetually open mouth. Her consistent attire is a maid outfit featuring a white apron, red ascot, tights, and Mary-Jane shoes, with bloomers underneath; her design varies slightly, appearing more human-like in the anime and rounder in the manga and OVA.

Functionally, Flandre exhibits superhuman strength and durability, enabling her to wield heavy objects like broken tree trunks, lances, chainsaws, and telephone poles as weapons. Her several-ton body weight makes her vulnerable to sinking in unstable terrain such as sand or mud. She requires regular recharging via electricity, often using a Borg-style alcove or a power cord accessed under her skirt, resulting in high household energy costs. Her eyes feature infrared vision with a German-language heads-up display (HUD), and she is a skilled driver of various vehicles. A self-destruct protocol activates if Hime dies, potentially failing if Flandre's battery is depleted.

Personality-wise, Flandre typically displays minimal emotion and communicates primarily through the word "Fuga" ("Hooba" or "Huga" in translations), a trait shared among royal androids except Ciel. Despite her stoic demeanor, she demonstrates occasional emotional capacity, like tearing up when blamed for Hiro's death in the manga. She exhibits childish tendencies: imitating cartoon characters such as Kerberotte, collecting dead animals for Hime, and retaining sentimental attachments like a seashell gifted by Ciel. When a loose screw causes malfunction, her personality shifts to rebellious and mischievous, marked by a smirk, sharper eyes, and sunglasses.

Her relationships define key aspects: unwavering loyalty to Hime sees her assist in combat, chores, and transportation. Interactions with Reiri involve cooperative combat despite mutual annoyance, particularly when Reiri must carry Flandre during power failures. She maintains a friendly dynamic with Hiro, even after inadvertently deflecting a truck into him, causing his death in the manga. A significant relationship exists with the android Ciel; she repairs him and receives a seashell symbolizing freedom, which she cherishes after his self-destruction. Flandre also has familial ties to other Frankenstein androids, including an older combat-specialized sister model, Francisca, who serves Hime's sibling Sherwood.