Familiar Strangers

"I used to find the phrase 'my past haunts me' a bit dramatic. A little over the top, especially for mundanities.
But now, my past is sitting by my window. Haunting me.
Literally."

Celia Greene watched Bennet March die.
It's not something she'll sugarcoat, and it certainly isn't something she'll forget. But she readily welcomes distractions in her life. Anything to keep her mind off of what she witnessed, what she couldn't prevent.
So when her mom offers her an opportunity to reconcile things with her absent father and grandmother, her first instinct is to decline. It sounds more like a chore than a reunion, and she's got plenty to deal with.
Namely, the ghost in her room.
Bennet March.
When it becomes increasingly obvious that Bennet isn't going away, Celia sees the trip as a chance to figure things out. One month to answer all her questions.
Why has Bennet returned? What does he want? Why does her grandmother 'need' a runway?
Most importantly-why can a random passerby see Bennet?