Yousei-san refers to the mysterious and whimsical fairies in the anime series 'Jinrui wa Suitai Shimashita' (Humanity Has Declined). These fairies play a central role, acting as agents of chaos and innovation in a post-apocalyptic world. They possess advanced technology, magical abilities, and a childlike curiosity, often causing unexpected problems and comedic situations. Despite their cute appearance and playful demeanor, they are unpredictable and serve as a reflection of humanity's desires and follies, deeply influencing the story's events and protagonist's journey.