The Eldest Lady's Personal Bodyguard

Due to his sister's divorce, the protagonist fell into despair. After being drunk, he met an old man who claimed to be the King of Killers. He became his disciple. After practicing hard for several years, he accepted the task given by the old man and sneaked into Country X to assassinate the leader and rescue hostages from a certain armed force. But the heir to the King of Devil Killers, who is so awesome and does not blink an eye at murder, actually wants to be a bodyguard for a wealthy lady in the secular world? Isn't that too ridiculous? This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.