I've nearly beat the game. I've already been inside the last maze leading to the climatic confrontation. A sneaking suspicion told me my characters weren't powerful enough to defeat the big boss--that's what they call the major opponents in these games--so I decided to go online and find a cheat sheet.
Boy, oh boy. I missed so many little side quests! These side quests provide incentive for completing them, lovely little goodies to help your characters. Weapons, armor, and various other items of usefulness can be found or won. So, cheat sheet bookmarked and open for easy reference, I started to backtrack. I'm glad I did. I unlocked another fusion monster for Kurando and have powered up several other characters.
Of course, having the cheat sheet means I now realize just how many things I've missed and how much more powerful my characters would be if I had only known! Ah, well. It's just a game, right?