An excerpt from The Adventure Time Encyclopaedia: Inhabitants, Lore, Spells, and Ancient Crypt Warnings of the Land of Ooo Circa 19.56 B.G.E. - 501 A.G.E. compiled by His Lowness, Hunson Abadeer, Lord of Evil:

FINN THE HUMAN

Teeth: A few missing due to his imbecilic propensity to chew on rocks, tree trunks, and dead lizards.

Eyes: From a distance, dungeon black. Close-up, frozen-cow-udder blue. When he’s hypnotized, his left eye turns green, so don’t hypnotize him if crazy eyes freak you out.

Hair: Absurdly long and absurdly blonde. When he removes his hat with a flourish, his gender becomes an impenetrable mystery.

ATTENTION, GIANTS & FLESH-EATERS!

If you find Finn the Human, do not eat him! Remember, he might hold The Secret to Everything. Without him, you might end up a carcass that looks like Burnt Toast, the type you have to scrape with a knife. (Then again, Finn might know nothing–zero, zip, nada– and you’re out a nice meal. Tough bananas, O Giants.)