The protagonist, Nagisa Rokudo, died once and was reincarnated.
It wasn't an isekai reincarnation or a modern fantasy reincarnation, but a place that closely resembled the world where he originally lived.
Though he felt slightly disappointed, Nagisa had an existence known as 'childhood friend' which he had always desired in this world.
She was cute, beautiful, smart, and the epitome of a perfect childhood friend.
However, she fell ill and couldn't move her body properly.
But by some stroke of luck, a miracle happened, and her childhood friend recovered and became healthy.
And — without anyone knowing — she transformed into a yandere, harboring a demon within.