If it's a man or a woman... but I guess "shrinking violet" could hardly apply to a man...

So... in a context of love, there is this idiomatic expression "vierge effarouchée", but that has often a sarcastic connotation...

Another version, though, without the sarcasm, would be this one : "biche effarouchée"
Type it in Google, and you'll find many instances of this one...

For a person, that is living its life in a shy and hidden way, one could say : "c'est une petite souris". (Link below)
http://baka-nezumi.over-blog.com/56-index.html

To note "biche effarouchée" would apply better to a cute woman when "petite souris" would better apply to a little old lady or a child...