Seishiki ni Majo ni Natta Nidome no Akuyaku Oujo wa, Mou Futatabi to Taisetsu na Mono wo Ushinawanai to Kokoro ni Chikau @comic

Princess Alice, who was turned back to her 10-year-old self with the "witch's power" before she died, meets Theodore, a heretic (Nox). He is the most hated and shunned member of the Knights' Order, but for a girl who was killed because of her crimson hair, he is the only person who can share her loneliness! However, contrary to the rumors (of the past), she is intelligent and gets caught up in a battle over the right to inherit the throne...?

"Would you like to become a witch's knight out of self-interest?"
"That's a ridiculous invitation, I don't think you're a princess, is it?"

A relife story of a figurehead princess and a lonely knight who finally meet, both of whom have "crimson"!

(Source: TO Books, translated)