Unpredictable as weather
She's as flighty as a feather
She's a darling 

She's a demon 

She's a lamb

She'd outpester any pest
Drive a hornet from its nest
She could throw a whirling dervish out of whirl
She is gentle

She is wild
She's a riddle 

She's a child
She's a headache 

She's an angel
She's a girl