She is the only E-00 series robot Doctor Eggman built with a female personality. She has many skills such as walking through walls and knocking characters out without touching them. She was defeated in the end when Hawk dumped water on her, causing her to become paralyzed and short-circuit. Taking advantage, Knuckles hit her with a super punch that sent her flying into the sky, destroying her.
A robot based on a female ninja, E-91 was equipped with shurikens, gliders, an electrocution net, a chain with a weight at the end that could either be small or turn into a wrecking ball, and an entire arsenal of different Ninjutsu techniques. Her name is a pun; the word "Kunoichi" means "Lady Ninja", but when the word is separated, it also means "91" (Ku-no-ichi). The Lady Ninja was dispatched to retrieve the red Chaos Emerald, but ran into trouble in the form of Knuckles the Echidna and Rouge the Bat. Eventually, she had Knuckles on the ropes, but Knuckles' human friend Hawk dumped water over her, causing her to become immobolized. Taking advantage, Knuckles hit her with a super punch, ultimately destroying her. Lady Ninja was 120 cm (4 ft) tall and weighed 200 kg (440 lb).
- Her head shape is similar to Bokkun's.
- She actually speaks when chasing down Rouge, saying, in a robotic female voice, "I want that Chaos Emerald!" And when she grabbed the Chaos Emerald from Hawk, she says "Thank you."She also apparently made a few grunt noises as well.