Kita is actually a devout person and he fell in love with Atsumu.
They met when the noble that governs the fief when they were inspecting the lands after a long time. Probably because the succession rites are already near.
Atsumu is on the verge of dying and Kita is so desperate that he prayed, every day, every waking moment, to beg god to please save Atsumu.
But the god refused, and Kita was devastated.
Kita couldn't do anything but weep beside his lover that is about to take his last breath.
Then the devil came, and proposed a deal with Kita. To do what the god can't, won't do.
To save Atsumu.
So without hesitation Kita agreed to partake a deal with the devil. Without even hearing the consequences.
The devil did what Kita asked of him to do. Saved atsumu from the brink of death. And Kita couldn't be more thankful.
But his happiness didn't last long when he heard the devil said the consequences.
Atsumu will not remember their time together. Not remember Kita.
Not their laughter under the shade of a tree. Not their promises made under the night sky where the moon and the stars are listening. Witnessing their vows.
Nothing.
Kita will have to serve the devil. Become a being that he was taught to hate.
And in that moment, when Kita felt everything crashing down, Kita was left with everything.
The feelings, the memories, the vows, the love.
And so Kita persevered. Worked hard so he can be sent in the world of humans. Just so he can glimpse his love. Living a happy life without him.
Laughing under the shade of tree with the town's people. Under the night sky, making a promise to the god that left him to die.
Kita took what he was allowed to have until he can't. It was an impulse. To appear in front of his love. And he didn't regret it.
Because Kita thought that being able to be this close to atsumu is enough reason to break the rules.
Where he can feel Atsumu's heart beating so fast that Kita is sure that he is alive. Close enough to see his reflection on his lover's eyes. Eyes so bright and so alive.
In that moment, Kita thought, he would do everything all over again.