SO I have acouple theories here the are
I think the reason Azula can bend blue fire is not because she is strong beacuse she is GIRL.
I mean Iroh is way stronger than her and his fire still looks red and we haven't really seen any girls firebend.
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I also think that Azula is not Zuko's sister cause I mean They don;t nothing at all and his never calls her "sister" and
Azula in her chidhood never really ??called her "MOM" or at least we never her talk about it. And she never really mention any about her mom either,
also Iroh has never ever referred to Azula as his neice....Always by her title or name. I read several fanfics that say that Iroh is actually Zuko's dad, and a few that say somebody else is.....A couple say that Azula is the one with a different dad.
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