Like with Pokémon Crystal & Emerald, Pokémon Platinum builds upon the story of its predecessor by adding various new aspects. One of them are various new characters which enter the game and have various different additions to the plotline. At time of writing, only two of the major new characters have been revealed and their true purpose follows the story and increases it considerably. Below is the data that we know of them;

International Police Officer: Looker

The first character of note is this police officer codenamed as Looker. He is investigating the goings on in Sinnoh; The Climate has got cooler and Team Galactic's operations have seemed to pick up. You seem to meet up with him at several points within the game, including at the Valley Windworks and Jubilife City. He will constantly be around whenever yuo have dealings with Team Galactic including when you hunt them down in Veilstone City and when you're climbing up to face them in the Spear Pillar. After Cyrus stays in the Distortion World, you will not see him again until you find the rest of Team Galactic's admins in Stark mountain. Here, he will finally arrest Charon as the rest of Galactic run away. While he does not battle against anyone, we do see that he has one Croagunk that he uses in Stark Mountain

Team Galactic Professor - Charon

Charon is the new member of Team Galactic in Platinum. First seen in the Valley Windworks, Charon is a vital part of Team Galactic's science divison. He does little else in the game apart from over-seeing Galactic's machines to hold the legendary trio in place. However, after you obtain the National Dex, Charon takes command of Team Galactic itself and goes to Stark Mountain with the rest of his commanders. He attempts to steal the Magma Rock from it in order to capture Heatran for uses in his new schemes. He is apprehended during this by the player and by Looker and gets arrested. Charon does not battle at all during the game and we do not know what Pokémon, if any, he has

When you enter the secret room in Galactic's Building in Eterna City, you find a diary entry that states that as a child, Charon befriended a Rotom that came out of his lawn mower while looking for a lost toy robot. They became good friends but Charon couldn't physically hug his new friend. When he tried, Rotom shocked him and made Charon faint. When he awoke, Rotom was found in his toy robot so Charon & him could play together easier. As such, Charon created various specific appliances for Rotom to jump in to as he grew older