Considering that there are 13 characters in total and that there are only 8 slots, I wonder which character is better for recruitment and which isn't. More over, there is the possibility that some characters when recruited will have negative effects in future recruitments, such as when one joins, another leaves or when one joins, another wouldn't join.
Here's a quick list for anyone who needs help recruiting characters. X3
Spoilers, obviously.
Claude:
Physical fighter. Compulsory, you either choose to play as him or he joins you in the first dungeon.
Rena:
Healing mage. Same recruitment conditions as Claude.
Celine:
Attack mage. Compulsory for Cross Cave dungeon, afterwards she will ask if you want her to join permanently or not.
Ashton:
Physical fighter. If you recruit Ashton, you cannot recruit Opera or Ernest. After rescuing the kidnapped children in Mars village, visit Hilton and talk to the citizens. Two men near the dock mention a dragon in the Salva mines. Go back to Salva, enter the mine, head west, and eventually you'll find a save point, and a bit beyond that, Ashton fighting the dragon. After some talking, you get the option to add Ashton to your team. Recruiting Aston unlocks a sidequest where you defeat the demon Xine.
Opera:
Physical fighter. If you recruit Opera, you cannot recruit Ashton. If you do not first recruit Opera, you cannot recruit Ernest. Before entering Cross Castle, do a Private Action in Cross to see an event where a three-eyed man is fleeing the castle. After the Tournament of Arms, head to the bar in Hilton, and Opera is introduced to you. She'll ask you about the three-eyed man, so tell her that you saw him. Go back to Cross and ask for another audience with the king. He grants you access to the Mountain Palace, which is located on the mountain between Cross and Clik. Head up there and explore the dungeon (which has some pretty tough enemies), find Opera, and she'll ask to join you.
Ernest:
Physical fighter. You cannot recruit Ernest unless your have recruited Opera. After recruiting Opera, go back to Arlia and do a Private Action. Go into the forest and you'll find Opera's crashed spaceship. Now continue on with the game as normal. After exiting the Hoffman Ruins, Ernest will appear and you'll have to fight a (fairly weak) boss before you get the option to recruit him. (I think if you decline his offer to join, Opera will actually leave your group, but I am not 100% sure about this.)
Precis:
Physical fighter. If you recruit Precis, you cannot recruit Bowman. Do a Private Action in Linga. If you're playing as Claude, go north to the item shop to find Precis. If you're playing as Rena, go to Precis' house in the southwest corner of the town. After a bit of talking, Precis will ask to join you.
Bowman:
Physical fighter. If you recruit Bowman, you cannot recruit Precis. As long as you didn't get Precis earlier, Bowman automatically asks to join you after you clear the Sanctuary of Linga.
Dias:
Physical fighter. You can only recruit Dias if you are playing as Rena. Dias automatically asks to join you permanently after you arrive on the Continent of El.
Leon:
Attack mage. You can only recruit Leon if you are playing as Claude. Same as with Dias, Leon asks to join your team permanently after you get to El Continent.
Noel:
Attack/Healing mage. After you complete the Cave of Red Crystal and defeat the boss, Noel automatically asks to join you.
Chisato:
Physical fighter. After you first reach Central City, head left from the Mayor's office to encounter Chisato. After that, you'll notice her spying on you a lot on your journey. In the Cave of Red Crystal, in the bridged section (just before the boss, IIRC), take the lower level to the treasure chest. An event occurs where Chisato runs by and drops her ID card. Pick it up, and after you clear the dungeon, go back to Central City and find her again. This time you'll finally get the option to recruit her.
Welch:
Physical fighter. You need at least four characters in your party to recruit Welch. After the Tournament of Arms, but before the Sanctuary of Linga, do a PA in Lacour. Head towards the castle and Welch should appear and ask to join you.
Phew, that took longer to write than it should have. D:
I'm a bit rusty when it comes to Star Ocean. >_>
I think the strongest team would be Claude, Chisato, Opera, and Rena (although if you're more confident than me, I guess its possible to switch out Rena for Bowman or another fighter).
Anyway, as to who I prefer. My first time through Second Story, I used Claude, Rena, Celine, and Leon for most of the game, but two attack mages turned out to be a really bad idea, so I switched in Precis, and Chisato once I got her. When I play through Second Evolution I'll likely be using Ashton or Opera, as well as Dias. (I seriously hope he's got a better physical attack than in SS. He was so slow and always ended up dying. ;_;) I'm definitely not making the mistake of using attack mages again. >_>
