Paul

Paul (Japanese: シンジ Shinji) is Ash's main rival in Pokémon Diamond and Pearl, appearing many times and coming off as more of an anti-hero, rather than a villian.

Appearance and Personality
His official artwork seems to be based on Silver's artwork from Pokémon Gold and Silver. His personality also seems to be based loosely off of Silver's personality.

If he thinks a Pokémon isn't strong enough, he will release it. The soul exception to this would be his first pokemon Turtwig, who is through-out the series, a fully evolved Torterra. He also appears to be overall unfriendly towards others, excluding his brother. He also seems to be self-rightous. In some fan art, Paul and Dawn like each other. Although this isn't very noticeable in the anime, it's possible.

Anime Appearances
Paul's debut was in Two Degrees of Separation, where he briefly appeared.

Paul then reappeared in When Pokémon Worlds Collide! where he battled Ash. The battle ended in a draw.

Paul's third appearance was in Different Strokes for Different Blokes, where he captured his Ursaring. This marks the first time a main character that wasn't part of the gang or Team Rocket captured a Pokémon on-screen.

Paul's fourth appearance was in Shapes of Things to Come! where he challenged Roark to a battle, winning the Coal Badge with the help of Elekid.

Paul stayed until A Gruff Act to Follow to watch Ash's Gym battle.

Paul didn't appear again until Top-Down Training where he battled Cynthia and lost.

Paul then appeared as a comic relief through-out A Maze-ing Race! where he was interrupted three times while trying to catch a Fearow.

Paul appeared as a main part of the story in the three-part Tag Battle arc starting at Tag! We're It! where he was paired with Ash in a Tag Battle at Hearthome City. In Glory Blaze, he released Chimchar, where Ash took Chimchar in. Finally, in Smells Like Team Spirit!, the last part to the arc, his Elekid evolved into Electabuzz and he and Ash won the Tag Battle contest, receivng a Soothe Bell. However, he gave the Soothe Bell to Ash.

Paul again appeared in Tears for Fears in Chimchar's flashback, where Chimchar revealed that Paul had not treated it right.

Paul reappeared in Riding the Winds of Change! where he fought and captured Gliscor.

Paul was in a flashback in Lost Leader Strategy! where it was revealed that he had beaten Maylene and earned the Cobble Badge. He had also left his Electabuzz with his older brother, Reggie.

Paul appeared in Enter Galactic! where he asked Reggie for Electabuzz so he could use it to fight in the Pastoria City Gym.

Exactly 10 episodes later, Paul made another appearance, this time in Pika and Goliath! where he was briefly shown, saying that Ash needed to evolve his Pikachu.

Paul made two brief cameos, one in Staging a Heroes Welcome! and then in Pruning a Passel of Pals!. There he watched Ash participate in The Wallace Cup.

In Chim - Charred!, Paul made another appearance where he battle Ash. The battle hadn't finished due to Chimchar's powers going wildly out of control.

Paul's 19th appearance was in Naetoru, Hayashigame... and Dodaitosu!?, an undubbed episode of Pokémon.

Paul's 20th appearance is in Pokeringer! Big decisive Sky battle!!

Trivia

 * Interestingly, Paul's hometown is Veilstone City, which is also where Team Galactic's base was set up. Also interesting to note is that, in the games, you can only evolve Murkrow by use of the Dusk Stone, which is only available inside of said base.
 * Even though Ash is the main character Paul could easily defeat Ash.