Oak and Ash - After Ash's mother came down with an illness and eventually died he was sent to live with Professor Oak. With memories of Ash's mother haunting them everytime they even looked out the window Professor Oak decided to take the job as the Professor in the Jade Region. Ash, wanting to still be the very best, goes with him and is determined to become the Regional champion in the Jade region. He will be one of your rivals throughout the game.
Barry - After revising his skills he eventually defeated Lucas/Dawn as champion and now travels the world. Simple enough :p
Brendan - He is your brother. He has been traveling the region and training for a long time now and is part of the Elite Four as the fire type trainer. Expect a challenge!
Sir Aaron - Sensing that the Silver Alliance is up to know good he comes to the region with his pokemon, Lucario and Pigeot to put an end to their misbehavior. He eventually asks for your help because he sees that you have what it takes to save the world.
Derrick - He's from the same town as you and he's your neighbor. He is your biggest rival in the game.
I just thought this stuff up. If anyone has better suggestions just let me know, but for now, I'll feel like a boss.
Hmm... as a fan of Lucario, I am glad that Sir Aaron has come to the Jade region. But that isn't sir aaron that is actually Riley from Dppt. He is a descendant of Aaron and uses Lucario too. I know this is very weird a suggestion, but why not recolor Riley to a darker version and give him a Zoroark?