Trash Cannon Fodder

[The Summoned Hero] was a novel about a man who was summoned from Earth to become a hero and defeat the Demon King.   From the beginning to the end of the novel, several characters interact with the main character. Agnes Whitlock, the main character's close companion and one of the Knights of the Order, was one of them.  Agnes also had a younger sibling. He was weak, ignorant, and vicious, and he made enemies wherever he went. So it was unsurprising that he went missing early in the novel. The problem was that the book's weak cannon fodder, who was killed at the beginning, later turned into a demon. And to make matters worse, he'd be killed by his own sister. "...fuck." He was reincarnated as Seth, Agnes's weak and trashy younger brother.