What are your character likes and dislikes and why? if your character loathes the colour brown, to name one example, what is the reason for it and does that loathing make them change their behaviour in any way? Could that loathing change the way the story ends because the character's actions change (and not necessarily for the better either)?
You almost certainly wouldn't use all of this in one story but listing what your character likes and loathes is a great way of finding out all about them before you write the story properly. You will write with a deeper knowledge of that character (and it does show through).
From the writers' viewpoint, I dislike characters who have no reason to be in the story but are clearly making up the numbers and I have come across this. (Earlier in my writing life, I've been guilty of it but I think many of us have been. The important thing is to realise the characters don't have a role to play and cut them out). Each and every character has to have a very good reason to be in your story. Each and every character has to be distinctive (even if it is, say, Character A loves flowers and Character B hates them because they have chronic hayfever. Easy to tell those two apart!).