There is definitely a strong theme of family and friendship in Freak the Mighty. What's nice about this story is that the family and friendship theme is not always a nice "feel good" theme, either. For example, Max's real dad is a horrible person. Kane killed Max's mother in front of Max. Max goes through the entire novel being terrified of his father and wanting nothing to do with him. Max hates the very genetics that he holds inside of himself. It's not a fun way to look at family, but I appreciate how the author shows that not all families are perfect and wonderful. Grim and Gram are Max's grandparents, so they are family to Max. They take care of Max and treat him well, but they are not the emotional core of Max or the story.
Kevin is the emotional anchor for Max. Their relationship clearly illustrates a theme of friendship. This story is not about having lots of friends. This story is about the importance of having a good, close friendship. Max and Kevin do not have a lot of friends. They are both looked at as freaks of various kinds; however, that is what draws them together. They are both broken, but together they are whole. They find ways to support each other, and they find ways to have fun together, too. When Kevin dies, Max isn't just sad because he lost his friend. He's sad because he feels like he has lost a part of himself. That's what the theme of friendship shows in this book.