Demon Slayer: The Second God Who Walks the Earth

Naturally born with markings + naturally translucent + complete Sun Breathing = Muzan's new father
(This is not a time-travel story; the protagonist is unaware of the plot.)
Chiba Yo, who had everything taken away by demons, joined the Demon Slayer Corps at the invitation of the Flame Hashira and became the Flame Hashira's stepson. From then on, all the demons faced their nightmare.
The protagonist is a descendant of the royal family and looks very similar to Yoriichi. He comprehends the Breath of the Sun on his own and creates his own sword techniques.