Author Rodman Philbrick explores several themes in Freak the Mighty. One is that of the Misfit in society. Both of the boys are outcasts because of their outer appearances; Max is abnormally large, while Kevin is diminutive because of his physical deformities. Additionally, they are both psychological misfits, Max because of his father's background, and Kevin because of his intellectual prowess and general weirdness. Another theme is that of the main characters' Transformation. Separately, the two boys are weak and vulnerable; together, with Kevin on Max's shoulders, they become the seemingly invincible Freak the Mighty. In addition to this unusual physical transformation, both characters learn to deal with their individual problems through their bond of friendship. After Kevin's death, Max's inner changes continue for the better. And there is the theme of Friendship between the two polar opposite characters. Their first contact is cold, but the two boys gradually discover that they both work well together and, more importantly, need each other's strengths in order to grow. In the end, they are both willing to risk their lives to help each other.
For Max, the "learning disabled" boy who has low self esteem at the beginning of the novel learns that he does, indeed, have value and merit. Through his experiences and adventures with his newfound best friend, Kevin (who has Morquio's Syndrome, and is deep denial of his impending mortality), Max learns to read, and eventually write his experiences, which turn out to be the book, "Freak the Mighty," itself.
For Kevin, our tragic hero, his flaw is his outward denial of his disease, which has left him stunted in growth, though his mind is far beyond those of his 7th-8th grade peers. He has strong transhumanist views, ultimately wishing to be surgically given a "new body." His fascination with knights and robots brings us a theme of human frailty and its need for mechanical improvement, which is ultimately ironic, considering Max's newly gained "strength" of mind and selfhood, the opposite of physical weakness.
Another theme of the book is that one does not have to turn out like one's parents, as evidenced by Max using his great size and physical might in only beneficial ways (despite some tantrums). He proves this to the community after he escapes his father. Max did have something to prove, and this frightening experience teaches us that we do have some control over our destiny. Another irony: Kevin did not. Hence his flight into his transhumanist delusion.