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powergo1 You get Vice Versa, the Houndour. Whilst with it's original trainer, Houndour is determined to show that not all houndours are as devilish as they seem, and thus will avoid using moves it deems as 'evil' - this includes all dark, ghost and poison moves, any move capable of causing a burn, as well as Frustration, Dream Eater and Rage. It also wants to show off it's lighter side, so must learn and keep Solarbeam in its movepool as soon as possible.
However, when traded to Heartgold, it gets so angry at being seperated from it's original trainer that it becomes aggresive and angry. It may only use moves in the above list (any moves not on the list cannot be used, including Solarbeam, Sunny Day, and any Fire move which cannot burn), and it will refuse to take any items from it's trainer - so it cannot be healed with items, nor may it hold items in battle. In addition, when switched in, it will refuse to switch out until it either wins or faints, even if the opponent sends out another pokemon.
In Platinum, Houndour must show off it's goodness by defeating at least half of any Team Galactic members pokémon each time they are encountered after he is caught. In Heartgold, he must show off his devilish nature by defeating at least half of any Gym Leaders pokémon solo for each he encounters after being traded over. He will also refuse to fight fellow hellraisers such as Team Rocket and the rival.
Houndour must be traded to Heartgold version after every gym badge obtained in Platinum, and can be traded back to Platinum after the next gym badge is obtained (again, in Platinum version).