Grillby is always wearing color-coordinated clothes. His style depends on the day, however; one day he might be wearing a suit, and the next he could be wearing a t-shirt with jeans. The flames on his head tend to be wild-looking, as if every day is a bad 'hair' day, and Grillby tries not to notice since he can't do much to fix it.
Grillby is usually laid-back, and enjoys being a social butterfly. However, if he sees something is out-of-line, he can become either very aggressive or very upset in the blink of an eye.
Grillby and Sans get along well. Sans likes to go to Grillby's to vent about the fights he has with Gaster, for the most past, but will also listen when Grillby has something important to say.
He and Gaster are close; Gaster comes in every day for a cup of coffee and some small chat.
Grillby likes to talk with Burgerpants while they work. Though the two aren't exactly 'friends', Burgerpants may occasionally go to Grillby for advice.
Grillby doesn't know Muffet well, but he will occasionally drop food off at her house because he knows how paranoid she is of leaving.