Grillby plays a minor role in NEO Madness, meaning this is a supporting role. Grillby is a fire-like monster from Snowdin, a monster of few words and even fewer emotions. He is a bartender that is very competitive when it comes to his rivalry with Muffet, another monster, for business in town. He works tirelessly all day in order to keep his diner the most popular in town. 

We imagine him as one of those super intimidating bartenders that doesn’t talk unless he’s pushed to a certain point. So when he does talk, he's always spitting fire. Figuratively of course. Grillby should have a very modulated voice, he should speak very clearly. He should have a very exasperated voice, as if he was tired of the situation he's in. He should have a very resonant tone, speaking very sharply (exaggerating the S in steep for example).