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Sweetheart (Sweetie Pie in the online book) or Sweaty, known as simply Sweetie, is a dog that Frank got when his dad told him that he ran over his old dog Nutty with his car. Sweetie is fond of Susan and Gramma just like how the pig is fond of Manny.
In Diary of a Wimpy Kid: Dog Days, Frank got angry when Grandpa told him Nutty was run over by the car and didn't run to a butterfly farm. Frank went and got a dog, which Greg wanted to name "Ripjaw" or "Shredder" but his request was not needed because they are "too violent", while Manny and Rodrick wanted to give him animal names. Susan then named it Sweetie (Manny in the movie), even though the dog was a male. Greg had problems with Sweetie, he would sleep on Greg's bed giving him very less space to sleep, and would bark whenever the commercial on gophers played on the TV because Frank once said, "Look at those Gophers, Sweetie." Later Rodrick was sent to get a bowl with his name, he got the name wrong, because it says the name "Sweaty" instead of "Sweetie". Later Frank gives it to Gramma since nobody is enjoying the dog.
Role in Cabin Fever
In Cabin Fever, Gramma has been over-feeding Sweetie and even giving him table scraps, and is now heavily overweight. Greg says that "you'd hardly know he's the same dog, as he looks like a tick about to pop."
Role in The Third Wheel
As shown in The Third Wheel, the Heffleys still have his exercise pen and whenever Susan holds a playgroup for little kids, Manny takes his toys and locks them and himself inside his pen.
In Hard Luck, it is shown that Gramma dresses him up like a little person, he could be seen wearing a suit, then Greg and Rodrick would also make raspberry sounds behind him which would make him look behind then go back to sleep, Frank tried it once but Sweetie farted on his face. Greg referred to him here as "a beach ball with legs."
Role in The Long Haul
In The Long Haul, it is shown in a flashback that Greg once ate some whoopie pies, but Susan thought it was Sweetie, and since chocolate is not good for dogs she had to get his stomach pumped.
Role in The Meltdown 
In The Meltdown, it is shown that when Gramma takes her dog Sweetie to a beach. He was shown lying down on the beach down south soaking up the sun.
- In Diary of a Wimpy Kid Online, Sweetie appears much smaller, with beady eyes, and a rough coat. He has a black nose, which is usually wet, and is a small breed. In the book version, his appearance is somewhat bigger, but mostly the same.
- In Cabin Fever, Sweetie had gained so much weight that he is depicted as having his feet hanging off the ground in the illustrations. According to Greg, he now looks like "a tick that's about to pop." Sweetie is mentioned but not seen in The Third Wheel.
- Sweetie also appears in Hard Luck, he is shown dressed up in a suit jacket and tie and described as "a beach ball with legs."
Sweetie is lazy and mischievous. As shown as he lies around all day, and when he made Greg chase him through the neighborhood in his underwear. Sweetie doesn't take a liking to most people, mostly for what they do to him, like Frank, who constantly seeks his attention and annoys Sweetie to no end. He usually dawns over the comfort of Susan, even though she doesn't like Sweetie as much as Frank does.
In the Movie
In the movie, Sweetie is a Labradoodle because it was mainly all they could settle on based off of his appearance. Greg is never really that fond of Sweetie.
In the Online Book
Sweetie doesn't look much different in the online book, since he is a dog, not a human. He is also quite small in the online book, about the size of a sock. It's unknown why he was so tiny in the online book. Sweetie plays a role quite similar to the one in the book. At Rodrick's party, some girls who are looking at the photo of the family with him in it say he is very "cute". Rodrick tells them that he is in the basement, Greg thought they were talking about him, until Rodrick took Sweetie and locked the door. In this version he is quite attracted to Greg's mother Ann (Susan).
On Wimpy Wonderland Island, in Rodrick's room, you can find a dog bowl that says "Sweaty" on it, like the bowl Rodrick got for Sweetie. This bowl is later used to distribute antifreeze.
In the Dog Days movie advertisement, go to the edge of the pool, and you'll see Sweetie next to the "Sweaty" dog dish.
- Diary of a Wimpy Kid Online (First appearance)
- Diary of a Wimpy Kid: Dog Days (First book appearance)
- Diary of a Wimpy Kid: Dog Days (film)
- Diary of a Wimpy Kid: Cabin Fever
- Diary of a Wimpy Kid: The Third Wheel (Mentioned only)
- Diary of a Wimpy Kid: Hard Luck
- Diary of a Wimpy Kid: The Long Haul
- Diary of a Wimpy Kid: The Meltdown
- Happy Meal Commercial
Sweetie after being overfed by Gramma in Cabin Fever.
- Even though Sweetie doesn't appear in The Third Wheel, he was mentioned and it was shown the Heffley's still have his exercise pen and when Susan holds a playgroup, Manny locks himself inside his pen to keep his toys to himself.
- In Diary of a Wimpy Kid Online he is way smaller than himself in the book, he is about the size of a sock.
- Greg’s description of Sweetie in Cabin Fever is that he looks like a tick about to pop because of Gramma overfeeding him.
- Sweetie has been mentioned in every book since Dog Days except for The Ugly Truth, although his role was much more significant in Dog Days.
- In the Dog Days film Sweetie didn't get give away to Gramma unlike in the books but in The Long Haul film he is nowhere to be found due to it being a reboot
- In the Dog Days film played by a Sweetie is a Labradooble
- In the books it is speculated that Sweetie's breed is a Beagle.
- Sweetie is a boy.
- Sweetie made an appearance in a Happy Meal Commercial created McDonald's along side with The Pig Rodrick Gammie, and Greg