Book One by a Betel Star Native

One day, the protagonist Lin Xi goes exploring and falls into a cave. When he wakes up, he finds himself in the world of Undertale. He makes many good friends there and relies on his experience playing Undertale to get by. He overcomes one obstacle after another and wonders whether he should leave or stay. Different friends, four different endings: he’ll reach them all. Here, he meets Sans and the others. Hahaha, Sans’s dry jokes! But he has a pretty high threshold for laughter. He meets Frisk and enters the world of Undertale, where he finds different endings and makes a life for himself.