From the beginning of Freak the Mighty, Max tries to distance himself from his grandparents:
Gram and Grim, bless their pointed little heads, they're my mother's people, her parents. (1.3)
Max tries to be super laid-back about it. They're just grandparents. Whatever. No biggie. But why would Max want to distance himself from the only family he has left? Are they bad people?
No way. These two are seriously sweet grandparents. The problem is that every time they look at Max, they see the man who killed their daughter (since he looks just like his dad). They love him very much, of course, but imagine having that daily reminder living in your basement.
And it doesn't help that Max doesn't want anything to do with them and prefers to hide in the basement. So Grim and Gram do what they can. They try to make the down under as nice as they can ("In the little room Grim built for me [he] glued up this cheap paneling" [2.1]), and they give Max the space he asks for.
Unbeknownst to Max, Grim and Gram's presence and influence are felt throughout the story. Namely through Max's good character and his continual references to things they have said: "It all goes to show, like Grim always says, that you can't always judge a book by the cover" (21.7). Even though Max tries to play it cool, it's clear that his grandparents are VIPs in his life.