What hormone is responsible for the expression of secondary sexual characteristics in human females?

Estrogen

The secondary sexual characteristics in human females include development of breast, widening of hips, increase in body fat and growth of genital hairs. A diagram follows :

The effect of estrogen in body : Here, we can see estrogen is responsible for all the secondary female sexual characteristics. Here is a small diagram showing the effect of estrogen on female (and testosterone on male):