eccentric young person (esp. girl); oddball girl
Colloquial, often affectionate or teasing term for a girl or young woman who behaves in a quirky, unpredictable, or dreamy way. The male counterpart is 不思議くん.
See also: 不思議くん
She's what you'd call an oddball girl—she'll suddenly start singing out of nowhere.
There's always one quirky girl in every class, isn't there?
A more general term for an eccentric person, without the youthful or gendered nuance.
From 不思議 (fushigi, 'mysterious, strange') + the familiar suffix ちゃん, often used for girls or young women. The exact origin is uncertain, but it likely emerged in casual speech to describe quirky young women.