Why Did I Have to Transmigrate as the Villain’s Younger Brother?

I found myself transmigrated into the fantasy novel “From Transmigration to Hero.” And not just as any character, but as Sean, the younger brother of the Villain Raizen—and the one who would eventually be killed by him. “An extra who’s already dead by the time the main story starts… seriously?” But despairing over it wouldn’t help, especially since Sean stood to inherit a fortune. So, I clenched my teeth and resolved to survive. [Quest: Call Raizen “Big Brother” three times or more: 30/3 Spend at least 30 minutes with Raizen: 120/30 Hold Raizen’s hand for at least 5 seconds: 0/5 Reward: Access to From Transmigration to Hero’s main text and Raizen: Bonus Chapter] Even with these ridiculous quests, I managed to complete them somehow. All for the sake of survival, I acted as harmless as possible, pretending to be the little brother utterly devoted to Raizen. But then… “You’ve manifested as a Recessive Omega.” Suddenly, an unexpected setting that didn’t exist in the original story appeared. “You said it yourself. That you and I are the only two people in this world.” Even after I barely managed to escape, the villain chased me down to where I was hiding?!