on Writers Helping Writers:

Whenever we write a protagonist who lost someone important within their backstory, we have some heavy lifting ahead of us. That “unseen” character—a character who has died or who is simply away for one reason or another—is going to need to be developed and brought to the page somehow to deepen the emotion beneath the protagonist’s loss.

It won’t be enough that we tell the reader our character misses that person, or how that we label how the unseen character used to make the protagonist feel—good or bad. That sort of writing reduces the relationship down to what we call “emotional abstraction”—outright naming the way someone feels, rather than letting the reader experience the emotion in a firsthand way.

If the loss or separation is truly crucial to the story of your protagonist, you’ll want to create the illusion of the unseen character so it’s as if we’ve actually met them.