Double Battles were introduced in the 3rd generation. In Ruby and Sapphire, you could only battle in Double Battles when 2 trainers were next to each other. In Emerald, you could battle with someone in Double Battles and trainers didn't have to be next to each other. In Diamond & Pearl, Doubles have again been updated.
In some areas of the game, you will meet some people that want to go through an area with you because of many different reasons. Each of them has 1 Pokemon with them and heals all of your Pokemon (yours and theirs) straight after the battle! In every battle you find, you'll have to fight with your new partner. So when you fight a trainer, it's a Double Battle, when you walk into the grass, you will find 2 Pokemon instead of 1!. When you do come across 2 wild Pokemon and you see 1 you want, you have to make the other Pokemon faint because you can only catch 1 Pokemon, the other must faint. But be careful, your partner might faint the Pokemon you want to catch.
When you are with your partner, it is the perfect time to train Pokemon. They don't take any experience from the fight, you always get healed, they help the fight and you Pokemon to KO at once! So you might want to use this time and gain an extra 3-10 levels with could help in the long run of the game.
Below are the trainers, locations and there Pokemon.
Eterna Forest
When you enter Eterna Forest for the 1st time, you'll meet a girl called Cheryl. She heard about Team Galactic and got spooked by it. She wants you to help get through the Forest. She has a Chansey that knows Egg Bomb and Softboild.
In the top right part of the 1st floor in this cave is a lost girl called Mira. She has a strong Kadabra with her. Since she is lost, just walk back the way you came to the entrance of the cave to save her.
Iron Island
When you reach a floor in the cave on Iron Island you will meet a guy called Riley. He looks like the guy from the 8th Lucario movie. He has a Lucario (which is such a surprise) so he can help quite well with the battles here. At the end of the cave, if you have a spare place in your team he will give you a Riolu egg! If you don't you can come back for it later.
Victory Road
After you get the National Dex, a new section of Victory Road opens up. When you walk into the new section a girl called Marley. She wants to get to Route 224. She has an Arcanine which knows Helping Hand (and other moves) to help you.
Stark Mountain
After you get the National Dex, when you go to Stark Mountain a guy called Buck will want to go trough with you. He has a rivalry with your rival to get to the top of the cave inside Stark Mountain and get the Volcanic Rock. He has a Claydol with him. When you get to the room, he takes the Rock and runs off. Talk to him in the Survival Area and he will return it. After that go back to that room and you'll find the legendary Heatran waiting for you.
