Kestra was a skilled Cloud Skim pilot from Trimont. She volunteered to search for Elian when word reached Trimont that he'd gone missing during his ill-fated sortie. She never returned. Her name is now carved on Trimont's memorial stone.
Kestra never appears directly in Skies Under Trimont, but her presence shapes it. She is part of why Elian is paralyzed by guilt and part of what eventually moves him to act. The chain is: Elian's original overconfidence led to the sortie that went wrong; the rescue that went wrong trying to save him cost Kestra her life; the prospect of Taller being lost in similar circumstances becomes unbearable specifically because of Kestra.
Mira was there when Kestra's name was added to the memorial stone. That shared history is part of what makes Mira the only person who truly understands Elian's state.
Elian's promise to himself — "I can't lose another one. Not if I can help it." — is directed at Kestra as much as anyone.
What goes here: Any additional detail about Kestra as a person — personality, history, what made her a good pilot. Even though she's absent from the story, a clearer picture of who she was sharpens what her loss means.