Hey there, corne cobbe! This question is for your OC who tries the hardest to not be noticed.
Q: Do they keep a lot of secrets or are they an open book?
so i think that's probably thatcher like, almost certainly. he doesn't like attracting unwanted attention whatsoever, and unless he's going out of his way to be social, he would really prefer people leave him alone entirely. attention usually brings questions, and those questions are usually either about things he doesn't want to answer, or lead to other questions about things he won't want to answer -- "Why are you drinking so much?" "Where did you get those scars?" -- so he typically just... tries to lay low.
and i think my explanation up there already touched on the answer to this question: yes, he absolutely keeps secrets. he is a book that is under lock and key -- the key is hidden, and he's left a note about its whereabouts in his living will, so that the lock will only open once he's gone.
this is also a bit literal? he considers his journal his legacy. it holds all of his story and some of those that unfolded around him -- the stories of people he's not sure anyone else remembers, and he wants to keep their memory alive in some way. he doesn't want anyone to read it while he's alive but he plans to leave a note on his body asking whoever finds it to read it and maybe take it to candlekeep because it contains information that could be important, useful, or dangerous and thus is best suited to being kept by the scholars there.
however, if he's smart, he'll start opening up... he really needs to start healing, and if he doesn't talk about it, he never will.