Personification is when a non-human creature or object is given human characteristics (think of the houseplant falling in Hitchhiker's Guide to the Galaxy and thinking "Oh no, not again"). Grendel is a monster, not human. But when Grendel attacks Herot,Grendel displays jealousy. Grendel is jealous of the joy and peace the citizens live in. Jealousy is a human trait.
