Grillby took Frisk to an extra room. "Not much, but it is a comfy bed." Frisk laid in the bed while Grillby made food.
Not that Grillby didn't observe that the officer was slow to leave the place and kept his eye on every interaction he made with Frisk. Monsters weren't trusted, he knew that.
Grillby held Frisk's hand while he was grocery shopping. Many people came up to Frisk, asking if they were alright.
"You are alright in his care, right sweetie?" A human asked. Frisk nodded. Another lady rumpled Frisk's hair, "Let us know if this man bothers you." Her husband also informed Frisk, "You have the right to fight back, okay." "Don't listen to that skeleton. Using you." Another man said and it went on and on.
"Grillby?" Frisk got his attention. "Yeah." "Why do people think I'm agreeable?" Grillby sighed, "Because you are." Truth was Grillby wasn't happy with many monsters.
If he was in Frisk's position, some people would be dead for sure. And the fact that Frisk was expected to not fight back angered him. This was a child.
Sure, they were the last soul, but that doesn't excuse hurting a child. Grillby knew this kid was human, he just kept it quiet to keep monsters out of the know from attacking them. He knew how many monsters were desperate to see the surface, but he never condoned violence.
Now, if it were an adult, it could be considered fair game to attack them, but a child is inexcusable. And they're lucky this didn't cause monsters to never be welcome on the surface in the first place.