I generally speaking keep a notebook on each novel I write for ideas. In there I include character profiles. I have descriptors I use. Traits I want to keep in mind for future reference. I keep it fairly simple and this is the method I have always used.
It is interesting to map out your characters in a little more depth though. Get into their minds a bit. Thing about their history. Their back story. Their philosophy. Their personality.
I found this profiler for your character consideration.
It might be a good exercise to engage in. Something I might consider. If not this particular profiler than something catered to the works I do and similar in nature.