To characterize someone is to describe them in a certain way, to try to pin down his or her personality. If you say your dad is stingy, you are characterizing him as a miser.
See the word character in characterize? Two meanings of character can help you understand this word. We all have character, meaning we all have unique personalities. Also, characters are people in books, movies, plays, and TV shows. Characterizing has to do with summing up how a person acts in real life or is portrayed in a work of fiction.