Avatar: The Last Airbender-Mariko (ZukoxOC)

"Shu met Oma on top of a mountain that divided their two villages, but because they belonged to different tribes that were warring, they were forbidden to see each other. However, their love was strong and they managed to find a way to continue meeting."
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Mariko was different. She was born with the unique ability to bend both air and fire and is the only dual bender in history. The odds had been stacked against her since she was born. Despite this, she grew to become the youngest master Airbender and Firebender, but despite this and having a younger adopted brother, she was lonely. No one knew about the constant war inside her head, with each element begging her to choose a side.
Eventually, she had to learn to ignore the internal war when she learned her younger adopted brother, Aang, is the new Avatar and she was destined to help him master Firebending, much to her reluctance.
One thing Mariko longed for most was for someone to come into her life and understand her. Little did she know, she would have to wait a hundred years for that person, who turns out to be the one hunting both her and her brother down, to arrive.