Idiom for a Shy Girl [closed]

Quiet as a mouse

(of a person or animal) extremely quiet or docile.

Wallflower

a shy or excluded person at a dance or party, especially a girl without a partner.

Shrinking violet/Modest violet

A person referred to as a shrinking violet is a timid or shy person.

Reserved

slow to reveal emotion or opinions.

Uncommunicative

unwilling to talk or impart information.

Source: Oxford Dictionaries

In a more general sense, you could use 'introvert', though this is not a specific term. Anyone who keeps things to himself and doesn't disclose his thoughts to many people is an 'introvert'.

The adjective Demure also describes a woman in this way:

(Of a woman or her behavior) reserved, modest, and shy: ‘a demure little wife who sits at home minding the house’; see, Oxford Dictionaries

You could use

reticent

Not revealing one’s thoughts or feelings readily:

she was extremely reticent about her personal affairs

Synonyms include:

Diffident

Modest or shy because of a lack of self-confidence:

Guarded

(of a person or a remark they make) careful; not showing feelings or giving much information

Taciturn

(Of a person) reserved or uncommunicative in speech; saying little:

(Oxford)

Wallflower

informal a person who has no one to dance with or who feels shy, awkward, or excluded at a party.