Find out what drives your character and come up with ideas for what could have induced that drive. Revenge = somebody was killed or something bad happened because of somebody else. Honor = maybe a failed victory or a past organization they belonged to. Regret = they failed at something or failed someone. What was the trigger or switch that made them the way they are? Psychology can help to identify the traits better so that you can understand what can cause them.
Whatever it may be, make sure it can also explain a characters personality traits and odd actions. If they're an alcoholic, explain why. If they only want to win or to get a wish, explain why. If they are always kind, explain what made them kind. What shaped them into who they are now? Lastly, make sure it matches with the story. If you find out a character is an ex assassin but assassin's haven't been utilized in the story yet or even explained then it isn't going to work with the plot well. Though of the character is in a romance story but hates romance because of a person leaving them on bad terms would fit perfectly with the story and plot as an antagonist or antihero character. I hope this can help.