Grillby, like most Underfell characters, has a darker and more evil personality than his Undertale counterpart and randomly screws up recipes for his customers. Unlike most monsters, he is friendly to Sans.
Grillby is often depicted purple in Underfell instead of orange, yellow, or red, the way he is in Undertale. He has sharp teeth and a black fur coat. Grillby's "Purple Flame" isn't canon at all, and some people say that Grillby should have a "Blue Flame" instead, so there is no actual canon flame color for Grillby's body.
- Underfell Grillby's name is inconsistent, and he is sometimes referred to as Chillby instead of Grillby.
- The standard purple version of Underfell Grillby was designed by ask-grillby / zoruui on tumblr.com