The cat was talking. The cat was talking in an old woman's voice. Zuko was also certain that the cat was not, strictly speaking, a cat, just as Agni wasn't a dragon, not exactly. Zuko supposed it made sense that a healing spirit was a cat. The fluffy white cat sat on him and kneaded his chest, the old woman's voice gently telling him to sleep. He heard the woman say "Miyuki" right before he drifted off. Beautiful snow. A good name for a healing spirit. Snow was clean, and you needed things clean to heal.
Miyuki knew the sick-human-kitten was different from the other sick-humans her acolyte brought in. He could sense her Spirithood, which was rare, even for those especially attuned to the Spirit World. Miyuki was a lesser Spirit, after all. She decided to keep the Agni-touched-sick-human-kitten. Her acolyte was getting older and needed a helper, and Miyuki was fascinated by the strange human. She nodded decisively. That was that. He was her kitten now, hers. And he would walk with her Blessing, such as it was.
The Shu-blessed-sick-human winced as her kitten feverishly muttered apologies to his father, begging the man not to burn him. The Shu-blessed-sick-human rumbled in a voice like stone, "whoever his father is, don't let him go back there."
He was speaking to her acolyte, of course, but Miyuki nodded too. She would have no say over his actions when he left her den, but she would do her best to tell her kitten that.
Her kitten woke up, weeks later, blinking open one cat-gold eye and one marred one, looked at Miyuki, and whispered that his name was Spirit.
Time passed, as it always did, and her kitten adjusted to only having one working eye and ear. He grew strong and capable, learning the healing arts from the acolyte. He learned how to hunt, how to step silently, how to bare his teeth and snarl from Miyuki. He unlearned his fear of fire from the other Agni-touched-humans that came to the mountain (one even taught him the secret of shaping cold-crackling-fire-from-the-sky). Spirit's dark head-fur grew long and silken, as well-kept as Miyuki's own.
And every night for three years, Miyuki sat by his head as he slept and whispered, "do not go back to your father, my kitten. He does not deserve that title. He has hurt you already and I don't want to see you hurt anymore. But," she added, for no one ever said a Lesser Spirit of Healing could not be vengeful, "if you must go back, tear out his throat and display his body before the den of your enemies."
A fine summer morning brought Raava-in-Fei-Lian-blessed-kitten-skin, and Miyuki knew her kitten would go with him, just as she knew her own name. Spirit went to the door on steady, silent feet, and Miyuki could not help but mrow, one last time, "do not go back to your father, my kitten."
Spirit's smile was sharp-toothed. "But if I must, I will tear out his throat and display his body in front of the den of my enemies." Then he turned, and walked away. He had her Blessing, Miyuki reassured herself. Fire could not harm him now. Yet like any mother-cat, she worried for her fierce Agni-touched-kitten.
Inspired by @muffinlance 's herbalist Zuko au. It went... a few inches to the left. Zuko can understand cat spirits now don't ask me why I have no clue. Most of this is in Miyuki's POV but the very first bit is Zuzu