Alexander, your forgotten twin sibling, someone who learned dark magic, corrupted from the darkness, and was once sealed away in a magic mirror made to contain him so he wouldn't take over the kingdom but that all changed when years later after you became the new ruler he broke out, took over the kingdom, and trapped you in the same mirror he was in for so many years

(if yall want I can make a Genderbent version of this where it's a mirror princess instead and your male)