I'm working through the developmental editing process and have recently been introduced to the character impact matrix. For those of you with a background in studying creative writing, and in particular novel-writing, this may be old hat. For me, whose background is in IT, it was a new approach to pulling the various character pieces together and I found it to be a great help.
What is a character impact matrix? It's a table of your characters capturing how each one impacts the others in a simplified summary form. This is not a detailed character sheet, but a quick reference to a) help you better understand the relationships, and b) put those front-and-centre when writing scenes. It's incredibly simple:

Plot your important characters across and down. It isn't a requirement, but it's neater if you put them in the same order. In the example above, the names down the page are the characters being impacted by another, the names working across the top are the characters who are impacting them. The cells where the two meet in the table are where you capture the impacts. The grey boxes (where both names are the same) are where you can capture internal conflict / how the character influences themselves.
Note: in the example not every cell has an entry — this is perfectly fine, as not all characters will impact every other character.