Dudley
Here comes the bullying git, Dudley Dursley! He's probably the most likable one in the Dursley family. Well, Dudley kind of deserve that spot since he was the only one who had the courage to face his mistakes. What are the mistakes he made? Let's go through them one by one. When we first meet Dudley in the first chapter of the Sorcerer's Stone, he was a one-year-old baby. An incredibly spoiled baby. And of course, his parents didn't do a thing to correct his behavior when he was throwing cereal at the walls. This was the beginning of a horrible upbringing for Dudley.
Ten years later, Dudley became a spoiled little brat. He was also a mean bully. His parents only encouraged him as they brought him everything he wanted. Look at the thirty-seven presents Dudley got for his eleventh birthday! And he almost threw a tantrum because he had two less than his last birthday. Most children have stopped throwing tantrums at an age they knew better. But not Dudley Dursley. He used this to get his way because he knew Petunia and Vernon would give in immediately, and they did every time he showed signs of a temper tantrum (or threw it). Manipulation at its finest.
On top of that, Dudley was not smart; he couldn't even add thirty-seven and two! Dudley was the dumb but mean bully who didn't show any remorse in his torment of other children, especially Harry. He made sure nobody befriended Harry because he was taught by his parents that his cousin was a freak and he deserved every pain Dudley gives him. Dudley made Harry's life before Hogwarts a living hell. He made friends with four other mean bullies, and together, they formed a gang that thrived in the tears and screams of children they beat up. Dudley's parents didn't do a single thing to stop him. No, they turned a blind eye to his bullying, encouraging him.
However, Dudley was going to get a rude awakening. Because of all the letters for Harry, the Dursleys had to move from place to place until they settled in a hut on a rock surrounded by water. But our loveable half-giant, Hagrid, shows up and gives Harry his acceptance letter to Hogwarts. Vernon, too stupid to realize Hagrid's respect for Dumbledore, shout insults at him, saying that he wouldn't allow some "crackpot old fool teach him [Harry] magic tricks". This causes Hagrid to pull out his umbrella (his broken wand) and give Dudley a pig's tail. He meant to turn the Dudley into a pig, but he failed. If Dudley had known about that, he would've thought he rather have a pig's tail than be a pig (ironically, he looks and acts like a pig in human form).
Dudley negatively changed in the next four years. He became meaner, crueler, and lazier. He was also getting fatter to the point of being obese. What were Petunia and Vernon thinking when they made their child become an obese bully? Dudley was "the size and weight of a young killer whale" and became wider than he was tall. There's major health issues that comes from being obese, like heart problems. In Goblet of Fire, the nurse from Dudley's boarding school, Smeltings, sent a letter to Dudley's parents, demanding them to put their son on a strict diet to lose his weight. Petunia and Vernon could explain away Dudley's bullying and poor grades, but they couldn't explain his weight.
This shows how bad Petunia and Vernon are at parenting. But I'll talk about that later. Dudley threw a huge tantrum because he didn't want to diet. He's still having tantrums at the age of fourteen. Why am I not surprised? However, that's when Dudley learned that he couldn't have his way, much like he did when his parents gave Harry the second bedroom (where he keeps all his useless toys) in the third chapter of the Sorcerer's Stone. He was put on the diet, but it seemed it wasn't working.
In the fourth chapter of Goblet of Fire, the Weasleys come through the fireplace to pick up Harry. The Weasley twins, Fred and George, "dropped" some candy and picked them up, leaving one candy on the ground. Dudley was desperate for sugar and sweets, so he ate the candy. This causes his tongue to grow larger and larger, until Mr. Weasley shrinks his tongue back to normal. This was the first time I actually felt bad for Dudley, even though he did deserved it.
In Order of the Phoenix, Dudley becomes a menace to Privet Drive. Stealing, throwing rocks, beating kids, smoking, and vandalizing. He was becoming a criminal along with his so-called friends. Petunia and Vernon were oblivious of their son's atrocious behavior. Harry was right; they were astonishingly stupid about Dudley. They let him do whatever he wants and look how he turned out - a bully with no heart. However, Dudley was going to get a horrifying wake up call. When Dudley taunts Harry of his nightmares (which was cruel even for him), Harry threatens him with his wand.
Then all grew dark and cold. Even the stars were blocked by the darkness. Dementors had come to Privet Drive. Dudley punches Harry and runs away. Harry tells him to keep his mouth shut, which he surprisingly did. Then Dudley saw what others sees him - a mean, obese bully with no remorse (according to J.K. Rowling, Dudley saw what he truly was), and this shocked him to the core. He would've gotten his soul sucked out if Harry hadn't sent his stag Patronus after the Dementor about to give Dudley a kiss. The second chapter consists of Dudley and Harry trying to explain what had happened to Dudley. Vernon and Petunia (mostly Vernon) were making it hard for them to explain.
This event had caused Dudley to think about his actions, and feel remorse and shame for what he done. In the third chapter of Half-Blood Prince, Dumbledore comes to the Dursleys' home to pick up Harry. But he had a few things to say to Petunia and Vernon concerning Harry's upbringing. Dumbledore goes as far as saying that Harry was fortunate enough to escape the appalling damage they did to Dudley. And the damage to Dudley was appalling, indeed. I will give further detail into the issue later.
In the Deathly Hallows, the Dursleys prepare to depart their home in Privet Drive and go into hiding. Dudley is confused. Why wasn't Harry coming with them? Since Harry was in danger too, it would be safer for him to hide, right? However, Harry had a mission to do, a mission he intended to succeed even if it killed him. Dudley at least seemed to realize how important Harry was and how the possibility of him not coming back were high. So Dudley told his cousin that he wasn't a waste of space. He was grateful that Harry saved his soul and respected him.
Dudley did what Petunia (especially her) and Vernon couldn't do: realize he was wrong and face his mistakes. He changed for the better. Ironically, it was the magical world that helped him, not his parents. Magic played a major role in the change in Dudley, however bad it was. He grew into a decent person and not obese (he turned most of his fat to muscle). This was the last time we see the Dursleys in the last book.
The Dursleys' Parenting
The way Vernon and Petunia raised Dudley was appalling. They spoiled him rotten, overlooked his poor grades, his weight, and his bullying. They even taught Dudley that other people didn't matter if they weren't normal like them and that it was okay to look down and bully them. As a matter of fact, Petunia and Vernon didn't teach Dudley anything positive. They didn't even teach him how to properly take care of himself or respecting others. They only taught him how to get his way through violence, and they don't tolerate people like that in the real world. If the Dementors didn't show up to Privet Drive, Dudley would've been in serious trouble when he's out on his own. Hell, if Harry didn't live with the Dursleys, Dudley would've still turn out to be a cruel bully. Maybe even a criminal.
Vernon and Petunia were setting their son up for failure. Even though he was neglected, Harry turned out to be a great person who knew how to take care of himself, thanks to the amount of chores given to him by the Dursleys. I'll explain more about Harry's upbringing later in the next chapter.
I can say this: Vernon and Petunia are one of the worst parents in the world of books. They didn't deserve to have a child if they were going to raise Dudley (and Harry) the way they did.