Kevin and Max develop a symbiotic relationship, but at the start, it seems unlikely that these two would develop a friendship due to their glaring differences.
Max struggles in school and has a learning disability in reading. He describes himself as unintelligent. He lives in the basement of his grandparent's home (his mother's parents). His father is in prison and his mother is gone. He doesn't have a particularly close relationship with his grandparents, especially his grandfather, who he calls Grim.
In contrast, Kevin is incredibly smart. The first time readers encounter him in the novel, he has created an ornithopter that has become stuck in a tree. This is the first instance of symbiosis--Kevin can't reach the ornithopter, and Max's size allows him to reach it easily. Max is befuddled by Kevin's enormous vocabulary. Kevin's relationship with his single mom is much different than his next door neighbor Max's relationship with his kin. It is clear that Gwen dotes on Kevin, and Kevin adores her. Kevin has Morquio syndrome, which causes him to have a twisted spine, short stature, and many health problems. While Max is physically healthy, Kevin is physically challenged. Together, they become Freak the Mighty, with Kevin relying on Max's physical strength, and Max relying on Kevin's brain.
The two were in daycare together when they were very young, and Max remembers Kevin. Kevin becomes Max's reading tutor. They become Freak the Mighty when Max lifts Kevin onto his shoulders to see a fireworks display, and then the pair tries to outmaneuver a trio of bullies chasing them.
They go on to have many other adventures. Kevin is brave and daring but lacks the physical prowess to enact his bold feats befitting a chivalric knight. Max is introverted and prefers the safety of routine and his basement dwelling. He doesn't like conflict and would stay inside his turtle's shell if it weren't for the prodding of his only friend, Kevin.
Max is insecure because of his father's past, and fears he will turn out like Kane the Killer. Kevin wants to defy his present circumstances and get a robot body so he will be free to do all the things he wants to do in life.