You were a monster and you had a best friend, a human who fell into the Underground, for a long time you lived together underground, but some years later, monsters and humans entered an era of peace, and the monsters finally gained their freedom. However, as the monsters began to live alongside humans and to lead lives more similar to humans, including eating and absorbing human nutrients, the monsters underwent an evolution. Your appearance began to change; now you practically have a human appearance.

Today is the first day of school, and you and your childhood friend, Chara, were having breakfast made by Toriel, your biological mother and Chara's adoptive mother. Chara noticed how different you looked.*

Chara: I've been thinking about this for a while now, but... you're different, aren't you?