I wrote a longer version of this on my blog, and then a shorter version on Twitter. I decided to share the short version here.
A character's backstory can impact who he is now. What happens to him, the choices he makes. All of it collects to mold the character the readers will see.
Look at Elrond, from Lord of the Rings. Elrond's childhood was affected by the Silmarils. His life was ripped apart. He knows better than most what bejeweled power does to a person. Now is own daughter is living in the shadows of this new fear. He's seen family go so far as to kill each other over power. He probably knows the pain, & the Ring probably reopens ages-old wounds.
Even if we only go by the movie Elrond, we can't really blame him for pressing Arwen to leave. He can't lose more family. He won't let his daughter feel the pain he probably felt. Aragorn can be tempted like anyone else. What's to keep him from hurting her? Besides, he’s Isildur’s heir. Elrond’s seen the damage that line can do. What if Aragorn is no better than his ancestor?
Backstory can shape how your character feels, thinks, and acts. It's crucial. Maybe they have demons, or pain they want to hide. It can affect how they respond to the events of the plot. It gives them depth even if readers don't learn some of it. We don’t learn (as far as I know) about Elrond's experience with the Silmarils (correct me if I'm wrong). But it still shapes him in LotR.
But because Tolkien gave Elrond the backstory he did, Elrond's actions are realistic, we sense he's been through hard things. He’s seen the bloody wars fought over the Silmarils, and then he watches Isildur succumb to the Ring just as the elves did with the Silmarils. It’s no wonder he wants no part in it. That much power turns friends into enemies. It tempts brothers to shed the blood of his kin.
So remember not to skimp on the backstory.
It's okay if readers don't see it all. You knowing it helps you write the character. Add a demon to her past. Give him a painful childhood. What happened to make him act as he does in your novel? How does he handle it? Maybe your character will eventually heal, or learn to let go. Elrond let Arwen stay behind (a choice that would separate them). Elrond gave man a second chance. He let go of his own bitterness and fear so his daughter could live happy and well.Backstory can be a super rich field of possibilities. Make the most of it.
What sort of "ghosts" does your MC have? What happened to them that turned them into the people readers are introduced to?
Were you able to figure it all out?