>>7189248Yuuka is a surviving yokai/monster from one of the Undertale timelines in which Frisk got possessed by the demon Chara and killed everyone in a so called Genocide route.
Yuuka made it out of the underground and got through the power of Sans of this timeline ans his relationship with Yukari to Gensokyo, he also gave her half of his power left, this is the reason why he is so weak and easy to handle for an epic gamer at the final fight against him.
After Touhou 5 Yuuka changed the timeline of Gensokyo with the power of Sans she still had left.
https://www.youtube.com/watch?v=raSwkrhAagc 