Charlie is either Greg's maternal or paternal uncle.
Role in Diary of a Wimpy Kid
Greg describes his uncle as giving his known nephews anything they want. ALthough that's how Greg describes him, when our protaganist was 7, Greg wanted a Barbie beach house. Charlie instead purchased the first thing in the store with the word "Barbie" on it and just got Greg a doll. When the first book took place, Charlie gave Greg a picture of himself.
Role in Diary of a Wimpy Kid: The Last Straw
Charlie gave Greg a clothes hamper for Christmas. Greg describes it as "getting a chore for Christmas." Greg asked Charlie to exchange it for money and give it to Greg so that he could buy something he would use. Susan says that the idea was "excellent" and that Greg would have to do his laundry form now on.
Role in Diary of a Wimpy Kid: Dog Days
Charlie is briefly and scarcely even mentioned. Greg mentions that Carlie has a recliner that he spends most of his life on.