Character backstories can sometimes be glossed over when you’re designing a character, because you get so tied up in the now, but really, the past and present of your character are far more closely linked than you might realize. You can’t just skip backstory, because it’s an integral part of who your character is now.
Past and present are interlinked in a number of places in characters, but there are three sections that I think are the most closely tied together, and those are nature, fears, and relationships. Sometimes you even end up tying this together accidentally. For instance, in “worst childhood memories” for my character Cordain, I found out that his sister Braia was bitten by a spider when they were little and he’d been terrified she was going to die. As it turned out, I’d already put down “spiders” as a fear of his, and now I knew why. And of course you can also do this intentionally, by looking at something about them (a fear, a goal, a scar, a mannerism, just about anything) and figuring out why they’re scared of that, what happened to lead to that goal, how they got that scar, etc.
Past relationships affect (at least to an extent) how characters react to people. For example, if your character had an abusive ex, they’re likely going to be cautious around people who seem interested in them. If they’re from a peaceful city where everything runs smoothly and everyone gets along the majority of the time and then they go to a big city that’s more rough-around-the-edges and has criminals in the back alleyways, they’re more likely to be gullible and end up hurt, but they also might be more able to avoid falling in with the wrong crowds.
And that’s another thing to think about with characters: Almost everything has a flip-side. Something like courage can be bravery or recklessness. Humility can be a simple lack of pride or self-deprecation. Think about what your character’s strengths and weaknesses are and then figure out how those traits can manifest themselves positively and negatively. Think about why a trait is more likely to show itself negatively or positively instead of the other based on how they were raised, past experiences, etc. What might they have been trained in or out of? How does their society look on a certain trait of theirs and how does that affect how it manifests or how much?
Then, what goals might your character have based on their past? Nissa, from The Shadow Raven, wanted to be a teacher when she was younger because of the hope and encouragement she had gotten from teachers growing up; She wanted to be able to give that same hope and encouragement to others like her. Now she couldn’t care less about that dream, but it was there for a while and had a basis in the past.
Past events are basically plot points in a longer story of a character’s life. Just like plot points within a novel tend to be catalysts for character growth and motivation, the past does the same thing, it’s just not always shown to the reader. The past has more value than you might think, so don’t leave it behind.