She looks exactly like Sister Bear, except she has green fur and yellow-green eyes. (In the book, she also has little horns on her head.) She manipulates Sister into riding Brother's new bicycle, which Sister is too small for, to give Sister what she wants and at the same time torment her. She pretends to be friendly with Sister, but doesn't actually care about her safety. She is not actually real, but a figment of Sister's imagination personifying her envy of Brother's new bike.
One night, she appears to Sister while sister is asleep and convinces Sister to take Brother's bike and ride it. As soon as Sister mounts the bike, the green-eyed monster pushes Sister, sending her off on a bike she cannot control. The bike gets bigger and bigger until it breaks. Sister then wakes up, realizing it was just a dream.
In the cartoon, she manipulates Sister to actually get on the bike, but as Sister's legs are too short, she cannot reach the pedals, endangering Sister by having her ride a bike she can't stop. Luckily, Brother rescues her in time.