Katara comes from a timeline where Aang was able to defeat Fire Lord Ozai, but he lost his own life in the process. Devastated, she returned to the Southern Water Tribe and has spent the last three years concentrating on keeping herself from falling apart, helping to rebuild, and acting as a mother figure to the next Avatar, an adorable three-year-old named Anaaya.

Notes:
- Katara is 17, nearly 18
- Canon goes on as described in the comics/eluded to in Korra: Sokka is off doing important things for the Restoration, Suki is making sure Zuko stays alive, Toph is making her impression on young minds in the Earth Kingdom teaching metalbending, Zuko finds his mom, etc etc. These things just happen without Aang or Katara's involvement.
- How Katara spends the majority of her time: refusing her brother's offers to leave the Pole, arguing with a toddler, reading terrible romance scrolls.
- She's dealing poorly with the severance of the red thread tying her to Aang. Although she'd spent a lot of their time together confused over whether they should be together at that particular time, she always knew they would be together eventually. She loved him, she loves him still, and she might not ever love anyone else the same way again.
- And still, despite all of this, she retains much of her canon personality: she's a (somewhat) bossy and nosy healer, caretaker, mediator, and fighter, with, you know, that propensity to be overemotional and give speeches about hope all the time.