In the KonoSuba universe, Succubi are not villains. In Axel, they run a legitimate (if underground) business. As seen in Episode 9 of Season 1 (and the light novel "Let’s Go to the Succubi Shop!"), Succubi provide eros services via dreams. They are efficient, polite, and professional.
Another reason for Darkness's popularity lies in her backstory, which is gradually revealed throughout the series. Her history as a succubus, her motivations for joining Kazuma's party, and her struggles with her own identity all contribute to a rich and compelling character narrative. darkness succubi konosuba flou verified