More than often, Frisk toured through the subterranean with Master Gerson and his student, a child-figure even, Rex (Monster Kid). They decided to take care of the Frisk and raised them as their own child. He brought them back to New Hope palace to see the former emperor, Asgore, and the empress, Toriel. Much later after that accident, they fell into the subterranean and met Master Gerson Boom. Note that there are certain spells that are permanent and you should seriously consider everything before you use them. The damage was irreversible, not even a healing magic could recover their eyes.
However… they didn’t know how to use them properly, so an accident occurred that led to them losing their eyes and their ability to see. One day, Frisk noticed that they had magical ability. The same can be said if there’s too much dark. Without dark, there’ll be too much light and that is definitely not a good thing. It just means it helps with balance between dark and light. It’s not actually bad thing despite having a “dark” character in it. Frisk happened to be a born mage, a dark mage to be specific. Mages still exist in underswap, even until present day. If you do the genocide route, he’d say something bizzare like “EXISTENCE… EVERYTHING IN IT… WILL DISAPPEAR COMPLETELY… WE WILL ALL… FALL INTO ABSOLUTE DARKNESS… WHERE THERE WILL BE NO MORE SCREAMS… SUCH DESPAIR… WILL NOT BE FELT ANYMORE… FOREVER…” He might tell you about his strangely accurate prediction if you ask him about what he sees one day from now.The Gaster Followers and most of the Waterfall NPCs swap.I suppose this can be said the same to Alphys before she participated in a battlefield. Thanks to Gaster, Asriel grew up to be an alpha geek. Gaster taught Asriel a lot of things when he was still a kid.
He’ll sell you his goods and allow you to talk to him, too. Don’t worry, he’s genuinely a good person and a cool uncle-figure to Asriel. Otherwise, the CORE (from UNDERTALE) wouldn’t have been made. But in reality, he’s really just an old monster with eccentric behavior and happens to have insanely high intelligence, which he uses it to contribute to monsterkind’s need most of the time.