In terms of knowing the character the least then it is Brook. Oda has shown us either part of or the entire childhood of each Straw Hat with the exception of Brook.
Before his stint as a pirate, the only info we have on Brook is that he served in some nation’s army. Apart from that, nothing. As far as I can recall, we do not know what island he is originally from, how he acquired his love of music, how he learned to play music, who his mentor was, why he wanted to become a pirate etc. etc.
One could say that Brook’s background is pretty bare bones and that he could be hiding a few skeletons in the closet.