Narcissism: Frequently blame-shifts, is a serious Attention Whore, and is somewhat self-righteous.
OCD: Bluntly refuses to use a towel that was used once by someone else, wear other people's clothes, and feels the need to wash his face after being kissed. Doesn't go in the ocean because marine life pee in it.
Sociopathy: Frequently sees to it that Rowley takes the fall for his antics, plays any number of physical tricks on Rowley, and is rather selfish and narcissistic.
Butt Monkey: Let's see, he's constantly getting bullied by his older brother, and most of his attempts to "look cool" backfires horribly.
Companion Cube: One Christmas, Mom got him a baby doll named Alfredo which he became extremely attached to until he lost it (actually, Dad took it and hid it). Then he replaced it with a grapefruit that he took care of in the same manner for the next 3 months.
Cool Loser: Subverted. Greg wants to be seen as such by others (the reader included), but most will tell you that Greg is a dork.
Deadpan Snarker: Considering that he is the narrator and his personality, this isn't too surprising.
Generation Xerox: In The Ugly Truth, it turns out that Greg looks exactly like Frank's cousin, Terrence. He's not happy about it.
Jerk with a Heart of Gold: He has his moments, notably in Cabin Fever where when Rowley anonymously snitches on Greg for accidentally vandalizing the school, he doesn't pull Rowley into it and takes the punishment by himself.
Loser Protagonist: Our protagonist here is an underachieving dork who completely fails at romance.
Love Redeems: In the movies. Take a look, in the 1st movie, he was a big jerk like in the books, in the 2nd and 3rd movie, he was more likable. Holly Hills was also introduced in the second movie, coincidence?
Middle Child Syndrome: Rodrick frequently bullies him and gets away with it, while Manny makes himself a pest but Greg can't do anything to him without getting into trouble with his parents.
Mr. Imagination: In the books he often fantasizes about being rich and famous.
Nominal Hero: Not particularly noble to begin with, and most of the time he only looks out for himself. But he is capable of correcting his wrongs in the end, if only out of guilt.
Not So Different: From Rodrick. Both have a high opinion of themselves despite not having much to boast about, neither does well in school, both are lazy, indoor types who doesn't have much interest in sports (much to their father's annoyance), and both are bad with romances.
Dogged Nice Guy: Okay, so "nice guy" might be a bit of a stretch, but when you see him try so hard to win the love of Heather Hills in the 3rd film, only for her to respond with rudeness and a cold personality, it's hard not to feel a bit sorry for him.
Dreadful Musician: Greg doesn't really think much of his music. But then again, Greg doesn't think much of him at all.
Flat Character: In the books, there's not much to him other than being Greg's tormentor. In the movies, he gets a bit more depth.
Karma Houdini: Subverted on one occasion when his parents ground him for throwing a party while they were away after he did everything in his power to prevent himself from doing so.
Kick the Son of a Bitch: Does this unintentionally to Heather, whom he was trying to win over. However when Greg turned up the volume and effects, Heather's birthday party is ruined, her dress is messed up. Heather is a complete asshole to the main characters, and the destruction of her sweet 16 is fitting punishment for her behavior. You can actually cheer on Rodrick for doing this, jerk he may be.
Not So Different: In the movies to Greg. Lets make a comparison; Rodrick has a high opinion of himself, is bad with girls, an underachiever in school, and lacks talent at the one thing he thinks makes himself cool (being a musician).
Ambiguously Gay: Sometimes, Rowley's actions are a bit suspect, especially with Greg. You'd think that a 12-year old boy would realize that sharing a "Best Friends" locket (in the shape of a heart) with another 12-year old boy would be weird or realize that being a fan of a rather effeminate pop singer isn't exactly manly. It could be argued that he's just completely unaware of these things considering that he is a Cloudcuckoolander and is completely sheltered, however.
Butt Monkey: At times, mostly as an indirect result of Greg's actions.
Character Development: Throughout the books, Rowley learns to stand up for himself and grow a spine instead of blindly following people.
Clueless Chick Magnet: He doesn't seem to be all that interested in girls, yet in Rodrick Rules he's managed to socialize with some of the most popular girls in his year.
Dumbass Has a Point: The very few times Rowley calls out Greg for being unfair, he's completely right.
Fat and Skinny: He's the fat and dumb fall guy to Greg's skinny straight man persona.
Companion Cube: The remains of his old knit blanket, which he calls "Tingy."
Enfant Terrible: Played for laughs in book 6, where he steals supplies and leaves the rest of the family for dead during a blizzard. Greg throws out the possibility that Manny was just throwing a tantrum and didn't honestly realize he was endangering anybody, but he's not 100% convinced that's the case.
Karma Houdini: He can get away with messing with Greg's stuff solely because Mom and Dad baby him. This gets ridiculous in book 6. He gets away with the events described in the above trope. That's right, he left the family for dead during a blizzard, and didn't get punished.
Kick the Dog: He left his family for dead during a blizzard to ensure his own survival. When Manny went missing for a day, his mother, Susan, found him in his room surrounded by food, water, toys and a space heater.
No Social Skills: Even for a kid his age. He's so terrified of kids his age that his only friends are imaginary ones.
Parental Favoritism: His parents spoils him very much and lets him get away with things that Greg and Rodrick would get in big trouble for. See Karma Houdini above.
Youngest Child Wins: Only in that Manny winds up getting treated much better than his siblings.
Parental Favoritism: Towards Manny. She even went all the way from her workplace to his preschool to cut his sandwiches because Manny was throwing a tantrum about his sandwich being cut in halves instead of quarters.
The Pirates Who Don't Do Anything: Zig-Zagged. Greg's mom doesn't appear to have a job, but he mentions that she runs an article in the newspaper, and it's implied she might have been a therapist (but one for younger kids if anything) but is implied to go to school. Otherwise... she's not really doing anything.
What Does She See in Him?: One's a snarky, Jerkass, grumpy, bumbling dad who hates teenagers, heavy metal, video games and only wants to toughen up his middle child while get rid of his eldest child. The other is a former kindergarten teacher who's a gentle, caring, well-meaning Moral Guardian mother who dances to heavy metal, loves her family for the way they are, the only thing wishing for them to be is more like a family and genuinely tries to do the best for her sons (though it's often lost because of her inability to understand the fact that problems teens face can't really be solved with methods used to solve problems for kindergarteners). They're married.
Dad (Frank Heffley)
Played by: Steve Zahn
Jerkass: He's a bit of an asshole to his sons, and frequently forces them to engage in activities they dislike (i.e. sports).
All Love Is Unrequited: Greg's crush on her, at least in the books. In the films, it's implied she may like him, too.
Hair of Gold, Heart of Gold: In the film version, she's portrayed as a very Nice Girl. She willingly hangs out with "class losers" like Greg and Rowley, helps out in the Country club to teach children tennis, and later offers to teach Greg and Rowley.
Foil: To Holly. Both are beautiful blonde rich girls, and one of the Heffley brothers has a crush on each of them. The difference? Holly is a nice and sweet girl who deserves her popularity, while Heather is a complete and utter bitch.
Jerkass: She's a self-centered bitch who refuses to help others even while on lifeguard duty, and frequently stops Holly from being nice to Greg because she'd rather her help out in her Sweet Sixteenth celebration. It was almost nice to see her humiliated when her beloved party is ruined and she ended up covered in chocolate.
Kick the Dog: Several times. In her first appearance in the third movie, she runs over Rodrick's feet with her car when he tried to flirt with her. Then she is shown to turn away a little girl asking her questions about the Country Club schedule even though she's a lifeguard.
Played by: Laine MacNeil
A girl at Greg's school. Patty and Greg aren't the best friends, in fact they are deathly enemies.
Alpha Bitch: In the movies. She is the class president, and seems to abuse her power.
Girlish Pigtails: Subverted. Even though her hair is tied in pigtails, she is not at all girly.
Karma Houdini: She didn't get punished for yelling at the drama teacher, threatening to ruin her life unless she played Dorthy in the Wizard Of Oz school play. She also goes psycho on Greg when he is unable to sing in the middle of the play, and yet she doesn't get punished.
Spoiled Brat: Her mother in the movies is the head of the PTA, so she gets what she wants from the teachers.
Played by: Alf Humphries
Rowley's father, who doesn't like Greg and finds him to be a bad influence.
In the books he got Greg banned from the country club, likely because he can't take being criticized.
In the first book and the movie Greg tells Rowley in a "secret language" that Mr. Jefferson smells like woman when he was in hearing range Greg was kicked out and it's worse when you realize that with Greg smelling like a woman probaly means smelling like flowers.
Moral Guardian: Thinks that Greg is a bad influence to his son Rowley and doesn't like to see Greg around.