Cyrus

Cyrus is the leader of the main Antagonistic Organization of the region of Sinnoh, Team Galactic who wishes to recreate the Red Chain and use its powers to control the two legendary Pokemon Dialga and Palkia (and, in Pokemon Platinum, Giratina). This done, he wished to simply destroy the current world and reshape it, with himself taking the role of 'god'. He is 27 years old, as revealed in Pokemon Platinum by Looker.

One of Cyrus' most defining characteristics (in the games, at least) is his apparent lack of emotion. While some fans may insist that he had touched Mesprit as a child and had all emotions drained away in a matter of days, this cannot be possible due to the events atop Mt. Coronet (where almost every line he spoke ended with an exclamation point). Another reason this is not true is because he grew up in Sunnyshore City, as revealed by an old woman in the town. The geographical distance between Sunnyshore and Lake Verity, where Mesprit dwells, prevents such a rumor from being possible. In the anime and manga, this trait seems to be absent.

In Diamond and Pearl, he is first encountered in Celestic Town in at the enterance of the ruins, briefly introduing himself to the player. Later the player fights him at the Galactic Headquarters.

After the player defeats Cyrus announces that he is going to Mt. Coronet, he gives the player a Master Ball (claiming he doesn't need it), which you can catch any Pokémon without fail. (Ironically handing the protagonist the means to capture Dialga/Palkia)

In Diamond and Pearl, the Spear Pillar incident merely ended with the screen fading to white and all of Team Galactic disappearing. This trick has been used in many 'climatic villain escape scenes' of games past. In Platinum, however, it's a completely different story. Giratina appears in a black portal and simply pounces on and drags Cyrus into the Distortion World. After choosing to follow with Cynthia and making your way through the Distortion World, the player is greeted with a few brief encounters with Cyrus and eventually a battle.

After he is defeated for the last time, he leaves and is not seen again for the rest of the game.

Trivia
Cyrus may be Dawn's father, considering that they have some similar features. Although, this is not confirmed by Pokemon.