Dr. Gaster wanted children but he couldn’t exactly have any so he created a pair of twins and
Dr. Gaster wanted children but he couldn’t exactly have any so he created a pair of twins and gave them life, but their souls were very weak because the science is iffy so he decided that by giving them parts of his soul (Mad science and all that) they would grow strong. Eventually the transfer rips Gaster from existence and Sans and papyrus inherit whatever was left of him. -- source link
#undertale#wd gaster#alphys