The Grassy Wizard is portly and has long, wild, grayish-green hair. He wears a bright-green shirt and dark-green pants that appear to be made of grass. Two leaves are attached to his face, one to his bearded chin and one to his forehead. His nose is long and beak-like. He also carries a magic wand that glows green when it casts spells.
When the Grassy Wizard first sells Finn the grass blade, he hides his appearance by shrouding himself in darkness and wearing a black robe and opaque sunglasses.
He is similar in appearance to Forest Wizard, due to them both having plant-like bodies.
The Grassy Wizard enjoys cursing people but is not very good at it. He is angered when Finn accepts the Grass Sword instead of seeing it as a curse. His personality could be compared to that of Ice King's.
The Grassy Wizard can control grass. Presumably, he built his yurt in the marketplace using this ability. He is also able to curse objects with grass powers. For instance, the grass blade he gives Finn is extremely precise, but it also tries to attach itself to Finn's body and reappears in Finn's bathroom even after it is thrown in a river. When Finn and Jake try to confront the Grassy Wizard, they are assaulted by grass monsters whom he probably enchanted. When the two finally reach him, the Grassy Wizard attacks them with more cursed grassy objects before surrendering.
Remember, no take-backs.
Mow 'em down, my cursed grassy objects!
That's not how people are supposed to react to an eternal curse!