Ok my theory on how Diana could see her real self without herself realising it.. because Lia use her magic to heal Diana before on orphanage... When she used it the necklace crack and Diana could see Lia real self
there's also that past hint where Diana was attracted to Lia in their first life. I probably don't make sense what I mean is Diana probably saw Lia's true appearance even in their first life with how she's always been trying to get close to Lia (and Lia avoided her because that's what most children in the orphanage did).
And since Lia looked exactly like Diana's favorite princess book (same pink hair) maybe that's the reason why she's always been wanting to get close to Lia even before, people tend to find comfort in what's familiar in a place they don't know.
so basically she's the lost daughter of the Archmage, which explains her pure mana and talent in magic. And she does have pink hair (obviously with the hint they give when her depressed mom(she has pink hair and pink eyes) is desperately weeping for her lost child), and my theory is her past "master" is the same person who kidnapped her in her first life that's why he taught her the wrong theories in magic and betrayed her. He literally owns an information guild, meaning he could keep tabs with her biological parents (like collecting information about them searchkng for their child or give them false information) and when he decided not to hide her anymore, he killed her. Maybe I'm based but her cracked necklace was probably made by him too, to hide her physical features making normal people see her different but I still dk how Dana could see her real appearance when their talented mages couldn't.
I could be wrong too since this is just me rationalizing.