In Manarina, go to this room. See that weird looking thing there with big teeth and long arms in the left screen shot? Search it! There you will find Domingo Egg. And then to hatch the Domingo egg, you need to wait until you arrive to Poa (again) after you defeat Elliot in the field battle. Talk to the guy that Max is next to. He will put the Domingo Egg on something warm and out pops Domingo! He's one of my favorite characters in the game. Domingo is the only flying character that can use magic. He kicks so much ass! If you train him right then he'll be the character with the most HP, most MP, and best defense!
Speak to Kokochi in that room in the screen shot to the right. He will show you his invention which will end in a crash. He will fix this problem and joins you later in the game. When Poa (the moving town) leaves in the next chapter, Kokochi will fly down and joining the Shining Force. Surprisingly, this old man is one of the most strongest characters in the game. Heh, the place where he built his flying machine looks so... complex.
After the first battle in Chapter 3, you will end up in this small cave to get the Lunar Dew for Zylo. Search the walls of the cave and you'll find the "Sugoi Mizugi" which is a bikini swim suit for Tao. Put it in her inventory and next time you see Tao battle, you'll see that she actually has an ass under that thick cloak. During the Lazer Eye battle in Chapter 3, search the spot (shown in the screen shot to the right) next to the three Dark Elves. There you will find the "Kitui Huku" dress for Anri. Put it in your inventory and check out her new look next time she battles. But also keep in mind that these sexy new clothes don't change they way they look in the animated battle sequences. They only change the way they look on the battlefield. Damn, this would be a really cool secret if it would change the way they look in the battle sequences. It's boring just having their battlefield figures changed.
Visit the church when being in Pao for the first time. If you stand near the priest you will spot Yogurt for the second time. He wakes up from his nap going crazy (as seen in the animated gif up there) and making weird whistling sounds. When this happens he automatically joins the Shining Force. If you manage to make him gain a level, he gets the Yogurt Ring. All that it does is make whoever has it look like Jogurt. Jogurt is a useless character who's purpose is just comic relief. You gain the "Jogurt Ring" when Jogurt fights for the first time. Whoever equips this ring looks like Jogurt when they fight in the animated battle sequences.
Search the note shown in the screen shot to the right. It's pinned to a house in Prompt. Here you can read that Musashi, a samurai, will help you beating the dark armies. Read this note after you have been imprisoned and Musashi will automatically join your army. If you try to read the note before you're imprisoned, nothing will be there. He's one of the most strongest characters in the game! Actually, I think he is the strongest character in the game.
Near the town entrance in Runefaust, you will notice a bush with flowers. Search the bushes and you will find Hanzou. This ninja is the last hidden character in the game. He's pretty cool. He's pretty strong and has a nice selection of spells. I usually don't recruit him since you get him too far towards the end of the game.