Both boys are social outcasts. They are outcasts for different reasons, but they are still not part of the popular "in" crowd. Max is huge. I mean really big. It makes him stand out when compared to everybody else his age. He's also not very smart. He's always calling himself a moron or butthead or something like that. Lastly he stands out to others because he looks just like his mother-murdering father -- Killer Kane. Kevin is physically the complete opposite of Max. Intelligence-wise he is the complete opposite, too. Being the crazy smart, tiny kid has the same effect on Kevin, though, that being big and dumb has on Max. Kevin is an oddity. He's seen as weird. The fact that he is always sick and then later dying doesn't help fellow young people buddy up to him either.
Both boys love using their imaginations. It's how "Freak the Mighty" came to be. Together they are invincible and their imaginary adventures together prove it. Both boys love learning, too. Max would deny it, but Kevin is able to get Max to open up and learn to read better and begin doing better in school. Both boys care deeply for each other as well.