"Genki" is Japanese for "energetic" or "enthusiastic."
The Genki Girl is a character — usually a schoolgirl, but not always — possessed of an over-abundance of energy, such that she runs everywhere (often with arms waving wildly or outstretched like airplane wings), speaks quickly (sometimes unintelligibly so), and always does everything fast, fast, fast! She's filled with confidence and determination, regardless of whether she's competent or not. Although usually played exclusively for comedy, sometimes the Genki Girl slows down for a serious or introspective moment. But not for long — she lives her life full-throttle. To sum it up, a good way of telling whether a female character is genki or not is to see if her family and peers are exhausted, astonished, or even creeped out by her chronic outbursts of vitality. (A female character is by far the more common version, but this trope is not limited to females.)
Despite what you'd think, the Genki Girl is usually not The Ditz. However, there have been a few blends. She is very often a Motor Mouth or a Nicknamer. Less sympathetic portrayals often make her the Jerkass of the group (though these are relatively rare). Alternatively, she could be both. Some are even the resident Cloudcuckoolander.
The Manic Pixie Dream Girl is a version of the Genki Girl who wants to introduce someone, such as her friend or beloved, to how to enjoy life. This version of Genki Girl will liven up your life whether you like it or not. A Manic Pixie Dream Girl uses her energy and love of life to help another character achieve the meaning of happiness.
If this is an RPG setting, she's almost always going to be the Fragile Speedster, thus fitting her personality.
If a work of fiction gives its characters symbolic flowers that represent their personalities, the Genki Girl will likely get the sunflower while her animal is the dog, though her energy won't necessarily make her go crazy.
Whatever you do, don't give her too much coffee, cola, or sugar, and definitely teach her that hard drugs are very bad. This is why she may be paired with somebody who is practical.
Voice actors sometimes become famous for just being able to keep up the role.
Compare Fist Pump and Hot-Blooded. Compare The Pollyanna for endless optimism and cheer rather than energy. Contrast Emotionless Girl.
See Keet for the childish male version of this trope.
Often seen performing The Glomp.
