The end of the alphabet has been fun finding what to post but Y is for Yesterday. All your characters have pasts. What happened to them in the years before the story happens has an effect on the present in the story. Yesterday can refer to what happened the day or days before the story begins and this is part of the characters pasts. Yesterday can refer to where they were born, when they came into existance and all that happened to them in their growing years. Now you may not want to start your story with the character's birth and upbringing but you must understand their pasts to write about the present and to look to the future. Now you may not want to write all these things but you should have ways to hint about what the yesterdays had to do with what they are today.
Maybe your character was rich or poor. These things have something to do what they have become. They may be educated or not. Also this will play in the character's development. Discovering the yesterdays of your characters will make for a stronger story. So think about the yesterdays of your characters and feed this inot the way they act and react to incidents in their present.