Pokemon Mystery Dungeon 1

These are the first games in the Pokemon Mystery Dungeon Series.  You are turned into a Pokemon, and it’s your job to find out why.  You and your friend form a pokemon rescue team, and the more rescues you do, the higher rank you get.

http://www.serebii.net/mysteriousdungeon/

Once you beat the main story, you can evolve, change team leaders, and gain access to new Dungeons.  Even when the game is over, there’s still a lot to do.