In my opinion backstories, personality analysis etc. for each (important) character should be written anyway. A short (1 sentence?) summary should be written for every single one, even the ones you use once in case you want to reuse them. You don't have to put it into the story, but it helps you a lot to feel the character while writing. And it's a lot of characters... I don't add any if it's not necessary. It's a story, not Facebook. ^.^
How the summary should look... well, it depends on the story, but for example speech manners, attitude towards specific characters, general personality type (MBTI?), a couple of most fitting adjectives, job... It's nothing firm.
It is sometimes recommended to write down everything like height, eye color, income, hobbies, dreams, hopes, political views, people to look up to, health, most hurting experience, attitude towards death etc., but of course this is mostly useless (note: just mostly, not always).