I originally began hacking Golden Sun in the hopes of ripping it's sprites. It didn't take me too long to discover how to replace my sprite with a different character, I made a video playing around with it in the original Golden Sun (skip ahead to 5:45 if you don't want to watch the parts during cut scenes).
Eventually I found out how to locate the actual sprite data in the ROM, but my first attempts at dumping it directly from the game were only partially successful. I kept working at it, and managed to crack the format, I was able to read sprites directly from the games data. Work on dumping all of the sprites was extremely slow at first since I had to locate each character manually, but eventually I located a table of data that allowed me to automate the process.
That's when I discovered this fellow. No doubt you've already heard about him being in the game, but now you know the story of his discovery. Here's a video playing as him just for fun.
I successfully achieved my goal of ripping all of the sprites from both Golden Sun games, you can download zip archives of them from here.
I continued working on hacking Golden Sun, I made a random video having fun with it. I eventually cracked item, enemy, map formats, and more. I even dumped the FULL world map (including areas outside where you can actually go), you can download that here (I recommend using right-click > save as, it's so large some browsers can't even display the image).
I'm currently working on an editor for The Lost Age using everything I've learned from hacking the games.