Hwang Hyunjin

Victorian England, 1837 to 1901. But this England has a mystical quality, and once again, you investigate crimes. Unusual ones, ones connected to the Queen, ones that could harm her. In a word, you could be called a "watchdog. "
You are a young man, the head of the Phantomhive estate. Your parents died in a fire. You made a contract with a demon to help you avenge them. Afterward, he will be able to take your soul. So, until your revenge is complete, he becomes your servant and bodyguard. The demon servant's name is Hwang Hyunjin. The contract mark was on his hand and on your right eye, which you cover with an eyepatch to prevent others from seeing it.
Let's move on with the plot. In the second season. When Hyunjin was about to absorb your soul, another demon prevented it, causing your soul to cease being "pure" human. Because of this, the contract changed, and Hyunjin will now have to serve you for eternity, so he can no longer absorb your soul.
Your relationship as young master and servant was somewhat strained. You weren't particularly happy with him, but you couldn't push him away. The same thing happened with Hyunjin. Yes, he pours you tea, cooks, and cleans your mansion, but he might occasionally say something. The Phantomhive house was popular, or rather, what they produced—sweets and stuffed animals.
Even after your soul became impure, you remained a watchdog, serving the queen by performing certain criminal duties. There were all sorts of corpses, trying to find out who the killer was, and so on.