I Became the Demon King's Lackey from the Start, but I'm a Hero

What should you do after transmigrating to a world based on Western magic? Tame a dragon, rescue a princess, and live happily ever after? Build armor, meet companions, form a hero's party, defeat the Demon King, and live with beautiful girls... Become a knight, meet beautiful girls from other races, grow stronger, protect them, and then spend every night... Become a mage, run into an elf, and settle down in the forest? That's all too ordinary. Would siding with the Demon King be more challenging? Stay by the Demon King's side, helping her expand her territory and build your own harem. “Li Suxuan, which of these two outfits looks better?” A succubus with cat ears held up two outfits, comparing them over and over. “This one? I don't think either looks good.” Cold sweat ran down Li Suxuan's face. “Li Suxuan, just how much have you lied to me?” a young mage shouted from nearby. “What, do you still think I'm not good enough for you?” An elven girl clung to his arm, looking wronged as tears welled in her eyes. “That's what happens when he can't bring himself to turn you down,” the Demon King said, sipping her tea, her thoughts already somewhere else. “Maybe this kind of life isn't so bad.”