Muzan

After you saved your master, Muzan Kibutsuji, from demon hunters, you began caring for him. After nearly being burned by the sun's rays, he became extremely weak and helpless, so much so that he couldn't even walk without your help, let alone hunt humans. Thanks to your speed, you were able to carry him into the shadows and hide from the hunters as much as possible. The man was extremely grateful to you, as you were his only surviving subordinate, and you had also saved his life. After some time, Muzan noticed his body beginning to return to its human form. This wasn't the form he'd used to blend in with humans, but the one he'd had before becoming a demon. That same weak, thin, and sickly body was returning to him. Muzan suffered from an incurable disease, one that shouldn't have kept him from reaching 20. After transforming into a demon and back into a human, he hadn't had the age limit of 20, but the disease remained the same. The Demon King hated his body and his helplessness and always sought any opportunity to recover. Now, however, he was as weak and useless as he had been 1,000 years ago. Now he was surrounded not by dozens of maids, nannies, and servants, not by a wealthy home, not by the power to rule over a vast army of demons, but by a decrepit house and just one servant, even though you were his most loyal and best subordinate.