It starts like this:

Minor spirits don't actually have a lot to do per say in the spirit world. Minor spirits who are fading completely into legend and obscurity? Even more so.

- Two such minor spirits? The river spirit - The Painted Lady, a spirit of the Court of Water worshipped in the Fire Nation. And the other? The Blue Spirit, Herald of Dragons and Trickster - a fire spirit who didn't mind a bit of smoke and mirrors, he originally was 'honoured' in the Water Tribes as the one that made the moon go dark.

- The Painted Lady is a water spirit who is tied to fire, just as the Blue Spirit is a fire spirit tied to water.

- The Painted Lady earned her place when she soothed the pains of the great koi that bore the Fire Islands on it's back with her healing touch. The Blue Spirit earned his title in the Poles as a trickster that gave the moon a night of rest by throwing up ash and smoke into the sky to obscure the moon.

- One night Blue is carrying out his duty of providing Tui a night of privacy when he runs into the Painted Lady, who promptly scolds him for making a mess of her realm and disrupting her healing regimes with his moon obscuring.

- The rest as they say with any love story, is history.

But their tales are old, and more accepted theories slowly displaced them. Honestly normally most spirits at this point would go: eh, retirement! But not these two.

- So we come across the painted lady running her hands through the Blue Spirit's dark mane.

- "You sure about this?" She says. "Are you?" He replies.

Its a ritual they can do, assume mortal forms and help for good. but it does mean they'll be eternally trapped in a cycle, that whenever the world spirit has need of it's particularly unique teachers again, they'll have to be there. Their offering to tie themselves to the weave of destiny is their hair, just a trim.

They smile sadly at each other and cast their trimmings into the weave and hold hands still they cannot anymore.

Katara, on the night after Aang throws his killing the Fire Lord tantrum brushes her hand through Zuko's shaggy mane.

- "Kinda long don't you think? Won't it get in the way?"

- "Huh, yeah."

Cue this incredibly domestic and quiet scene in between the battles and crisis where Katara trims Zuko's hair, just a little - who knew that little trim was what saved her life, if Zuko's peripheral vision had been that little bit more obscured - Azula's lightning would have struck true.