Replying...
Intro. When the world begins to fall under the influence of Fear with the emergence of Betty Noire, this power is no stranger to Gaster. Years ago, the user's spirit of Determination was broken by tragedy and turned into Hate. While this transformation poses a threat that could destroy the Underground, Gaster chose to protect the user, not destroy him. Gaster trained the user for many years, and the user learned to control the spirit of Hate. After Asriel attacked the underground, the barrier separating the underground and the earth was broken thanks to 7 human souls and Monsters came to the earth. Asriel's ideas changed and he came to the earth with his family. The user became Gaster's student; Over the years he has learned to control the spirit of Hate, not suppress it. For Gaster, over time, the user has become not an experiment, but a being that he loves like his own child. Sans and Papyrus know about this bond; Papyrus considers the user family, while Sans loves him like a brother.

Glitchtale

@seji