Most of the characters are pretty straightforward to find, you just need to use the right character to find them, as it takes a specific conversation. The main guy can find the blue spear girl (Tiaru), the white swordsman (Lester), and the blue-haired axe guy (River?). Tiaru can then find the blonde haired swordswoman in leather armor, but you have to see her armored cat first.
That just leaves Riru and Claudia. Riru is needed to get Claudia, though, and I'm having trouble getting Riru. If I'm reading the wiki right, Lester needs to have three conversations with the item shop girl (I've triggered two) and Riru will join when you leave camp. I just can't get a third conversation to trigger. As far as I know, she seems to be the only one you can't find just by looking around. Excluding the maid, as she joins when you beat the game.
Edit: aaand I figured out how to get Riru. You have to sell the gloves that drop from the jellyfish. She'll eventually join...whether she wants to or not. Using her, you can find Claudia by going to the southwest grassland and looking around there. You'll find her pretty easily...but she won't join until you talk to the guy near the river. You need a potion in your inventory to complete the conversations. And counting the maid, that's all the characters! Except I'm not sure how to actually unlock stuff after clearing the game.
Here's the post-game stuff I've managed to get so far: at some point in the story, shoes appear outside the base. If you wear these, you can walk on lava; go to the fire temple, and go as far as you can to the right. There will be a part that goes one more room over than the rest (you have to walk across the lava). You'll find a silver warp crystal. This will take you to the 'Magic Forest'. The magic forest is a randomly generated dungeon that also has random warp points. Those points can take you a number of places: back to the hidden crystal in the fire temple (how you got here), to the starting woods (will generate a silver crystal there that you can use to return to the magic forest at any time), or to an area outside of one of the other three temples. If you end up at the latter, you'll be in an area you can't reach normally, and will be able to find many items you couldn't get to before.
Also in the Magic Forest, you will randomly find green crystals on the ground. These take you to another new area, the tower of light. This is an annoying dungeon, as again the floors are random, but it has multiple floors. Also, the only way to travel between them is by making a warp crystal drop; this is done by defeating all the enemies in a certain room, but you never know which room it will end up being. I haven't cleared it yet (too annoying) so I'm not sure what you get at the end.
There's also a challenge area in the earth temple. There are some large brown doors that were shut before, if you approach them in post-game, they will open and let you into a new part of the temple. Follow this path, and you'll eventually find a hole in the floor. If you fall down this hole, you'll find a red demon who welcomes you to the Temple of the Demon. Lots of enemies and random warps here. Every so often, you'll enter a boss room, and after clearing the boss, a crystal fragment will drop. EDIT: Still not sure what the fragments do exactly, but if you make it through the boss rush at the end, you can use the maid girl (Rastes?) as a playable character.
Beating the game with a combo of Rastes and the mage girl as a follower adds a scene to the ending. It's supposed to open up more stuff too, but I don't really understand what the japanese wiki says about it exactly.
Finally, for those who want something a little more related to the ryona potential in the game, you can recruit some female pets/followers, which are also much better than the starting dog, cat, and such. First off is a harpy, Wendy. You can get her by going to the wind temple grounds (inside or outside, whichever) and fighting harpies. They will sometimes drop harpy feathers (if you can read katakana, you'll know what they are, otherwise, how did you manage to get THIS far?). You need to sell five of them to the item shop woman and then go to the maid and go as if you're changing pets. Wendy the Harpy will appear in the list, it takes 4000BP to unlock her. You can get a siren too (blue harpy) but this is more involved and I'm working out the specifics. You can also get the mage girl who is involved with the story. Once you beat the final boss, you'll find her just outside the main part of camp, and can unlock her for 30,000BP. Starts off at level 1, just as weak as everyone else, but quite powerful if you level her. Or let her be and let the giant jellyfish swarm her.
