Dialga is a sauropod-like Pokémon. It is mainly dark blue with some gray metallic portions, such as its chestplate, which has a diamond in the center. It also has various light blue lines all over its body. It has a somewhat wing-like structure on its back, and a crest on its head, resembling a type of dinosaur. Dialga also has two horns on top of its head, two fang-like horns around its mouth, long claws, and a short tail.
Dialga has the ability Pressure, which causes the opposing Pokémon to use twice as much PP for every successful hit.
Additionally, Dialga has the ability to control time. Dialga, like many other Dragon type Pokémon, can use many different elemental attacks. It is also one of the few Pokémon that can learn the move Aura Sphere. Capturing it gives you an item called the Adamant Orb, which powers Dialga's attacks.
Dialga is the only Pokémon that can learn the move Roar of Time, its signature move, aside from the special Event Pokémon Darkrai given in the Tenth Movie and in the Toys R' Us Nintendo World Tour, the Pokémon Smeargle, who can Sketch the move, Pokémon with Metronome, a Pokémon using Mirror Move immediately after Dialga uses the move, and a Level 100 special Event Arceus.
After chasing Team Galactic through Mt. Coronet in Pokémon Diamond, the player ends up on the summit, also known as Spear Pillar. When there, Cyrus will use the Red Chain he crafted from Uxie, Azelf, and Mesprit to call up Dialga. After defeating Cyrus, the Dialga seemingly waits to battle the player. As is with other Legendary Pokémon, this is not the only opportunity to encounter it. After defeating the Elite Four and Champion Cynthia, you may go back to the same spot and re-battle it.
In the Pokémon Mystery Dungeon: Explorers of Time & Explorers of Darkness games, and is treated as a male rather than being genderless, sharing this trait with some other legendary Pokémon. Primal Dialga's the main boss of the game and is found at the end of chapter 20. After finishing the main storyline, the player can choose to fight Dialga and have him join their team. Dialga's seen using Roar of Time on Sharpedo Bluff in the opening. In the beginning of the future, a silhouette of Dialga with glowing red eyes will appear in the Pit. Later he's seen on close-up. He uses Roar Of Time (In the same animation as the intro) to turn into something. Much later, you're knocked away. An awful form of Dialga — Primal Dialga — appears. He challenges you and you fight him as the final boss. He appears in his Normal Form after you revive yourself. He says he "seeks the repair of Temporal Tower". After the credits, he once again uses Roar of Time. This time to restore the player to the timeline as thanks for saving him from remaining as Primal Dialga forever. Dialga will eventually help your partner by bringing you back to life.
Dialga appears in the game Super Smash Bros. Brawl, seen in the background of the stage Spear Pillar. It interacts with the stage in various ways including slowing down time, distorting the stage itself, and attacking the players.
Dialga had a role in all the Diamond and Pearl movies with the exception of Zoroark: Master of Illusions. Dialga had a major role in the The Rise of Darkrai where it battled against Palkia in between dimensions. Dialga played a minor role in Giratina and the Sky Warrior where Giratina attempted to capture Dialga for polluting its world in its battle against Palkia. Dialga played a major role (larger than Palkia's or Giratina's) in Arceus and the Jewel of Life, where it saved Pikachu and Piplup from whirlpools at the ruins.
In the manga
In the manga, Dialga is being controlled by Cyrus and the red chain. Dialga uses its ability to control time for battling.
Dialga has no known gender, but in the Mystery Dungeon games, it is referred to as a male.
Dialga is derived from "Diamond", as Palkia's is derived from the Japanese word for "Pearl". It may also be derived from dial, referring to its time-based capabilities.
No other Pokémon has the same type combination as Dialga.
Dialga is the only Dragon-type Pokémon not to be weak to Dragon-type moves.
Despite being able to levitate, it does not have the ability Levitate and thus remains susceptible to Ground type moves (unless taught Magnet Rise by the move tutor in Platinum).
In the anime, Dialga's rear spikes grow whenever it uses Roar of Time.
Dialga is the only legendary dragon in the quadrupedal class in the Pokédex. Palkia, Reshiram, Zekrom and Kyurem are bipedal, Rayquaza, Origin Giratina and Zygarde are snake-like, and Altered Giratina has six legs.
In Pokémon Mystery Dungeon: Explorers of Time and Darkness, Dialga was the one who created the Hidden Land. Ironically, he is also restricted from going back there when on an adventure.
The fact that Dialga has to rest after he uses the move Roar of Time may refer to releasing the power of time and stopping himself from moving.