In Freak the Mighty, Kevin is "Freak." He is a tiny child, born with a birth defect that has left him crippled. However, his mental abilities are much more advanced than other kids his age. He lives only with his mother because his father abandoned them when Freak (Kevin) was born. His imagination and his heart (literally and figuratively) are too big for his body. He is the opposite of Max in every way except for the fact that they both have issues with their fathers and they both are "pure of heart" (to quote an accolade of the knights Kevin always imagines himself to be).
"Freak the Mighty" can be considered a character in this novel. Although it is a combination of both Kevin and Max, "Freak the Mighty" represents their friendship but also a conceptual character. Max is the physical component and Kevin is the mental component.