I guess

Can you explain how Athy became the other Aarondi? I feel like I accidentally skipped a chapter somewhere.

I read this from legal site and they call aarondi as arundel so i will use arundel instead

FL has red hair which is a trademark of an arundel (her red hair stands out in her hometown and no one has red hair like her even her family), but SPOILER

(DON'T READ IF YOU DON'T LIKE SPOILER)
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she's not an arundel

In the past, an arundel man got hurt and was saved by FL's father, the man felt thankful and transfer his power or something to the father (in this scene the father's hair turned red), that's why their first daughter has red hair and can use magic (the father probably lose his red hair after his daughter was born)