Here is a useful check list for character development.
This could be printed off and put into a folder for each book. You can add important changes, pivotal points in their lives and keep it for reference later if you’re writing a series where it might come up again.
When I play a character on my role playing forum I keep track of things like this and moments where another character impacts my character’s life. For anyone who is interested it’s forum based, so it’s heavy on story telling and typing. As a writer, I find it helps me be more descriptive and I find it entertaining.