She was imprisioned in the fire nation n she was specially born an air and fire bender. She was taken in from the Fire Lord after he saw her supernateral powers. The fire nation did millions on tests, injecting water bender and earth benders' blood, causing her to often get sick. Once complete, Midna became a fire, water, earth, and air bender. Fire lord trains her to be a weapon in order to catch the avatar, he also wishes for her and zuko to marry because he believes she is a wrong willed child. Just like Zuko, the fire nation took her mother away from her and her father now lives with the fire lord as his top general.