Let's start with differences between Max and Kevin, because those are more obvious. First there is the obvious difference in looks between Max and Kevin. Max is huge. I mean he is a really big kid for his age, and it makes him stand out. That doesn't help with his already self-conscious mind. Contrast that with Kevin who is small, and on top of that he looks sickly (because he is). Despite his small size though, Kevin has self confidence to spare, while Max sorely lacks self confidence. Part of that is because Max isn't that smart to begin with, but he makes it worse by not trying to hard. Freak on the other hand is "freakishly" smart.
Their attitudes on life are different as well. Kevin has an innate seize the day attitude, which is likely because he knows that he doesn't have a long life ahead of him. Max does not have that kind of positive outlook. If it were left up to Max, he would never come out of his "down under." Max flat out wants to distance himself from other people.
Despite appearing to be vastly different characters, Max and Kevin are alike in a lot of ways. Both characters are social outcasts. Both characters are social outcasts for the same reason too. They look "weird." Max because he's huge and resembles his murdering father. Kevin because he's so small and frail looking. He's described as being 3 feet tall and has to walk around on crutches. Even Max makes a snap judgment about Kevin. His initial description of him is a "crippled-up yellow haired midget."
Both Max and Kevin are broken characters as well. Kevin is physically broken by the disease ravaging his body, and Max is psychologically broken, because he witnessed the murder of his own mother. . . by his father.