The Burden of the Crown

Instead of turning her in, Lexa's parents hide her until she is ten. When she finally starts her training, she meets Costia who pretends to be a Nightblood. The two become rivals and eventually fall for each other.

[They entered Costia's private quarters. They were the same ones she had lived in as a child. Costia glanced around and blinked away tears that formed in her eyes at the sight.
"I had imagined our reunion differently."
"You could have been an ambassador for Azgeda. If you had thought ahead -," Lexa ignored Costia's sorrowful comment.
"I wasn't thinking ahead, Lexa. I was merely thinking about how I would survive the next day." Angrily, she turned to Lexa, "I was a damn child!"
"So was I!" the brown-haired woman shouted at her. "You have no idea what I went through while you were off someplace enjoying small town life."
"Is that what you think of me? That I was doing okay? Damn it, Lexa." Costia pulled off her top, exposing hundreds of scars that seemed to stretch all over her body. "That's how I was! So stop saying how bad you had it."]

or: Lostia enemies to friends to lovers to death

(The 100 fanfiction)