Lee, protector of the water tribe (book:water) (Zukka)

The Avatar has returned 100 years after he vanished, and although rumors claim he's nothing more than a 12-year-old boy who only knows airbending, the Fire Nation has prioritized hunting him down.
In some parallel world, Prince Zuko would be part of that hunt, but in this one, he died at the tender age of 12 when, still recovering from the Agni Kai against his father, he was attacked and killed in his royal chamber. A month later, Lee arrived at Pohuai Fortress, and three years later, the blue spirit helped the Avatar escape from the same fortress.

(Story can be found in AO3 in English (same title) and Spanish (Lee, protector de la tribu agua) versions)