"Kimi wo Aisuru koto wa Nai" to Itta Koori no Majutsushi-sama no Kataomoi Aite ga, Hensou Shita Watashi datta

Fiene, the daughter of a count, was neglected by her family. But she secretly leads a double life as a living legend, the most powerful witch in the kingdom! She marries the beautiful future duke, Sion, as a political move, but he declares that ""he will never love her."" While she was initially happy that neither of them would get in the other's way, she later learns that the one he actually loves is her witch form?!

(Source: Manga UP!)