Peekachu is a rather quite and calm Pikachu. However, Peekachu has a bad habit of stalking people nearly everywhere they go (even when they're in the shower, due to Peekachu being a pervert), and it is unknown why it does this. Peekachu first appeared in Pokemon Yellow, as the pesky Pikachu that stalks you, yet he didn't have an official name. He returned in the next game, where he could be seen on a random route on a day that is numbered 1, 4, 16, 24, 25, and 31. The player can't catch him though, and this can't even happen by hack. He didn't appear in Pokemon Saphire and Ruby, where he was said to be thrown into a cage by Professor Oak to be dissected so the old guy can know more about Pikachus. Later in Pokemon Pearl and Diamond Peekachu returns, but it is said that Oak had violently chased him into Sinnoh. By the time he reached there, he decided to stay in a vacant house. Peekachu appears in the last five routes, stalking the player once again. Oak continued chasing him all the way back to Johto where he appeared in Pokemon Heartgold and Soulsilver. However, soon he was replaced by other Pokemon. Eventually Oak kidnapped Peekachu and proceeded to do unspeakable things to him. 

To be honest .. I'd rather not.