(Author: This are snippets of Rinley's previous life so I used he.)
Rinley can be considered a geniusa. When he was one year old, he learned how to speak. When he was two years old, he started to read his masters book of herbs and alchemy notes. When he was five, he watched his master create pills.
His master's only regret was his crippled meridians. If it was acquired, it can easily be repaired but Rinley's situation was inborn. Rinley's master searched everywhere but can't find a cure. Thus, the task of taking care of young Rinley was given to his master's only follower which was his Flame Spirit.
The Flame Spirit knows nothing about taking care of children. Because of that, he often came to the mortal world when he has a chance to have a look on how they take care of their children.
So, when Rinley's master first came back from searching herbs in different realms. He was surprised with what he saw.
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Rinley was wearing a skirt.
Master: What did you make Rinley wear?
Flame Spirit: A skirt.
The master rolled his eyes. Of course, I know its a skirt. But why did you make Rinley wear a skirt, he's a boy!
Flame Spirit: That's currently the most trendy clothes on the mortal realm.
Master: ... On second thought, my Rinrin looks cute on a skirt. He's still young anyway so it's okay.
The master then secluded himself for two years to try to create a new pill that might cure Rinley. When he got out from seclusion, he saw Rinley wearing cat ears headband, his pajamas also a tail.
The master looked at the Fire Spirit.
Master: ... what's this?
Fire Spirit: Neko outfit, a new trend in the mortal realm.
The master thought, even though two years passed, Rinley doesn't look like he got any bigger. He is as small as when he was three years old. With his small frame and delicate face, he can be mistaken for a girl.
And that was how Rinley was brought up as a girl by the androgynous-with-limited-knowledge-about-sexes Flame Spirit until he was old enough to notice something and complain. As for what age is that, Rinley doesn't want the readers to know.
(Author: I wonder what mortal realms the Flame Spirit visited...)