Leave Your Spirit is a Avatar: The Last Airbender fanfic (though it is more The Legend of Korra themed) based around a simple idea: "What if Kya wasn't Aang's biological daughter? What if she was Zuko's?"
While Aang is away, Katara and Zuko end up sleeping together. Nine months later Kya is born. Katara doesn't think anything of it, and considers it pure coincidence, until two years later she notices a striking resemblance between her daughter and the Fire Lord.
Leave Your Spirit can be read here.
Leave Your Spirit provides examples of:
- Absence Makes the Heart Go Yonder: Aang is frequently away on trips, which leaves Katara open to cheating with their mutual friend Zuko. Katara still cares for Aang but his absence makes her romantic affections lessen over time.
- Alternate Universe Fic: It's a fanfic where Kya is secretly Zuko's from a fling Katara had while Aang was away.
- Chocolate Baby: Subverted. Kya takes after Katara in skin tone, hair color, and eye color, so there's no way to tell she isn't Aang's. Katara herself didn't even think twice about her parentage until she looked closely and noticed that Kya's nose, which doesn't resemble either her's or Aang's, looks like Zuko's.
- Death by Adaptation:
- Zuko's wife (presumably Mai) died prior to the story.
- Toph wasn't confirmed to be Walking the Earth at the time of the fic's writing, so the story has her as deceased by the end.
- Honorary Uncle: Katara and Aang's kids grew up seeing Zuko as "Uncle Zuko".
- Maybe Ever After: The fic ends on an open note that suggests that Katara and Zuko will have a DecemberDecember Romance.
- No Name Given: In order to keep the story canon-compliant, names are omitted for a few characters. At the time of release, it wasn't mentioned who Zuko married, who Sokka ended up with (if anyone), or what Zuko's daughter's name was, so they're all nameless.
- Related in the Adaptation: In this fanfic, Kya is biologically Zuko's daughter.
- Sex for Solace: Zuko and Katara get drunk and end up having sex for this reason, which Katara lampshades.
- Stress Vomit: After cheating on Aang with Zuko, Katara vomits more than once. It's too early to be morning sickness, so it has to be from the stress.
- Unrelated in the Adaptation: The fanfic goes with the idea that Kya isn't Aang's daughter.
- Your Cheating Heart: Zuko and Katara give into their Unresolved Sexual Tension, which leads to Kya. They avoid any more cheating until after Aang's death.