I have in my office a copy of a family tree, sketched by some relative many, many years ago, long before I was born. The tree shows long branches, cut-off branches, and a sturdy trunk, but the handwriting is so spidery, and everything is so jammed, it's hard to make out what's going on. Being a problem-solver, I thought I could untangle this branchy web with a diagram. Wrong. All I did was create more confusion. Then it came to me - I really don't care about genealogy, what I care about are the family stories. So I picked out a few names and did some research. Now that's fun!
I am doing the same thing with my current WIP. I have a family tree of sorts for my characters, but it's filled with their ages, heights, hair colors, who is married to whom and what they do for a living, etc. This background may never be used in the book, but it matters to me. This is part of their stories. I list their nicknames, their foibles, their loves, and what scares them silly. It's all linked on a neat sheet of paper, and whenever I feel as if I'm losing touch with a character, I refer to it.
It also keeps me from making that most horrible of mistakes, changing eye color in mid-book!