I don't know well about the differences between "modest", "shy", and "timid."
Native speaker’s answer
Rebecca
Good question. "Shy" is a pretty neutral term. However it can be used negatively. It implies that someone is uncomfortable being around new people but that they are able to open up and be more comfortable with time. "Modest" is a generally positive term. It means someone who doesn't think too highly or speak of themselves too highly even if they have reason to do so. It can also be a way of dressing: not showing too much skin and keeping a lot of the body covered, which is how it is being used here. "Timid" is a generally negative word. It is used to refer to a weakness of character. Someone who is "timid" is always uncomfortable around other people and does not open up with time. It is also used to refer to someone who is easily scared Ex: I sometimes feel shy when I first meet someone. Ex: I don't like to wear anything too revealing. I'm pretty modest. Ex: She's a pretty timid person. That's why she gets scared so easily.
