One of the reasons Star Ocean The Second Story R is so replayable is its extensive cast of playable characters. Putting aside Claude and Rena, who have to be in the party, there are a total of eleven characters whom you can recruit into your party. The downside of this situation is that you only have six slots in your party before it's full, so you can't bring everyone along.
This guide will help you with character recruitment in Star Ocean The Second Story R. In addition to providing details on how to find each character, the guide offers a bit of advice on whether or not each character is worth your time. Your team will not be crippled by taking any of these men and women along, but... some are better than others.
Celine
The first potential recruit is Celine. First met in Krosse City, Celine will force her way into Claude and Rena's party to explore Krosse Cave. Once you leave the cave you'll have a quick conversation with Celine, and you can choose whether or not you want her along permanently. If you turn Celine down she will rejoin Claude for a short time in Marze, though after that you won't see her again.
Celine is a backline spellcaster. She's good against random enemies, and not a terrible character overall, but in the long run you'll find her more useful for buffing your characters than doing damage. You probably won't want Leon if you decide to recruit Celine, and vice versa.
Ashton
Ashton is your second potential recruit. Proceed through the game until you reach Harley, the port town that will take you to Lacuer. Near the entrance are a sailor and an old man talking about a dragon in the Salva Mines. Head back to Salva, enter the mine, and run to the central area with the Unique Spot. West of here is the path to The Dragon's Lair.
Follow this path north. You'll eventually see Ashton running across a bridge, into the northwestern areas. Head north to find a large clearing, then go southwest to find a Save Point. Past here you'll see Ashton, er, get into a pickle. Offer to take responsibility for what happens to Ashton and he'll join the party. (If you turn him down you can try to run, but if you're too slow... well, Ashton catches you.) Check in with the owner of the Weapon Shop in Salva for some Twin Splicers for Ashton before you leave town.
It's important to note that if you recruit Ashton you will not be able to recruit Opera, and, by extension, Ernest.
Ashton is one of the game's stronger frontline fighters, and can serve extremely well alongside the likes of Claude, Dias, Bowman, Precis, and Chisato. Ashton's major downside is that his Luck score never improves, at least unless you put specific equipment on him, making Ashton terrible at Item Creation.
Opera
Another potential third recruit, Opera is first found in Hilton, after the Laceur Armory Contest. Enter Hilton's bar and Opera will ask about the three-eyed man you saw in Krosse. She'll leave after the conversation. Return to Krosse and speak to the king, who will point you to the nearby Mountain Palace. You'll meet Opera inside the Mountain Palace, and she'll join the party.
As mentioned above, Ashton prevents you from recruiting Opera. If you brought him onboard you can never recruit Opera, and vice versa.
Ashton is a short-ranged fighter with a lot of long-range Arts. She can hit very hard at a distance, so long as you keep her full of MP. Overall a solid character for your party... though you may not want to follow up on getting the other three-eyed alien, whom we'll discuss below.
Welch
A character who only shows up in the remakes of Star Ocean: The Second Story, Welch is found in Lacuer. She's available after the Lacuer Armory Contest, though only if you have at least four characters in your party already. Trigger a Private Action once you have the four characters, then head north, to Lacuer Castle. Welch will... appear... as you're crossing the bridge into the castle, and ask to join the party.
Welch is a frontline fighter, though she's fairly average in combat, and will be outclassed by most of your other party members. Where she shines is Item Creation, as Welch tends to learn a lot of Talents. On the whole she's one of the less-desirable characters to recruit, though as with every party member on this list, Welch won't cripple your playthrough.
Precis
One of two party members found in Linga, Precis is the second character who joins up via Private Action, though you'll unofficially meet her the first time you enter the town. Split the team upon entering Linga and either go to Bowman's Pharmacy, in the east (Claude only) or Precis's Home, in the west (Rena only). In either case Precis will chat with both party members and ask to join the team. If you agree she'll hop into the party when you try to leave Linga.
Precis plays a small role in Bowman's quest for the group. If you decide to ask Precis to join the party Bowman will not join.
Precis is a frontline fighter with a medium range, somewhat similar to Claude. She doesn't hit as hard as some characters, and her normal attack is a bit slow, but she has good Combat Arts and won't do much to slow you down.
Bowman
The other party member in Linga, Bowman will charge the party with fetching him a particular herb from a nearby cave. Once you bring him the herb - or any herb - he will automatically join the party, as long as you did not recruit Precis. If you did recruit Precis then Bowman remains in Linga.
Bowman is a melee combatant with a very short normal attack. That said, he can attack faster than just about any other character, and so long as he has lots of MP his moveset is pretty useful. A good candidate for a party member, though whether or not he's better than Precis is arguable.
Ernest
A character who's a long time coming, Ernest only joins the party if you've recruited Opera and visited the Sacred Forest in the south of Arlia Village. This will give you a look at Opera's crashed spaceship. After that, visit - and complete - the quest to the Hoffman Ruins. On your way out you'll find Ernest near the entrance. Fight the creature possessing him and you can invite Ernest to join the party.
(Star Ocean vets, note that you no longer need to return to Krosse to spot Ernest coming out of the castle. It happens automatically during your first visit. Handy!)
For obvious reasons, you can't have recruited Ashton if you want Ernest in your party. It's also important to note that if you decide to turn Ernest down, he will leave - and so will Opera. If you want Opera but not Ernest, don't check on Opera's spaceship in the Sacred Forest.
Ernest uses a whip, and hits relatively hard at a long range (for a normal weapon, anyway). That said, his attacks are rather slow, and on the whole inflict mediocre damage compared to other characters. Ernest isn't terrible, but he's not amazing, either.
Dias
The most-mentioned party member aside from Claude and Rena, Dias weaves in and out of the plot quite often, starting in Marze. He joins Rena during the kidnapping fiasco in Marze, and will fight Claude in the final stage of the Lacuer Armory Contest. You won't get a chance to recruit Dias until you reach the Lacuer Front Line Base, and even then he'll only join up if you chose Rena at the beginning of the game.
Dias is another solid frontline fighter, befitting his reputation. His normal attack is a bit on the slow side, but his Special Arts can rip groups of foes to pieces in no time flat. Dias is a great party member in general, so long as he's stocked with MP.
Leon
One of the more plot-important characters who aren't Claude or Rena, Leon first shows up during the Lacuer Armory Contest. He'll join the party during the trip to the Hoffman Ruins, then leave again once you're done there. Upon traveling to the continent of Ell Leon will ask to join the party, but only if you chose Claude as your party leader. If not, Leon remains on Ell.
Leon is similar to Celine in that he's a backline caster. His Symbology makes for some good magic, though Celine is just a bit better in that regard. That said, Leon's melee attacks are actually pretty decent at bringing down airborne enemies, as he doesn't need to jump to hit them. Still not an amazing character, but arguably better than Celine.
Noel
A reclusive resident of Energy Nede, Noel joins the party on their quest for a Psynard. He'll stick with you until you've tamed one, then (assuming you don't already have eight party members) ask to remain with the team permanently.
Noel is the second coming of Rena, but... worse. His spells aren't as effective, and he seems to have a slightly inferior melee range (not that you should be forcing your healers into hand-to-hand combat on Energy Nede anyway). If you desperately want a second healer then Noel is your man, but... yeah, you probably don't need him.
Chisato
A crack reporter with guts, Chisato appears several times during the early stages of your trip to Energy Nede. The first one that you need to trigger is in the Centropolis, if you head west immediately after leaving City Hall. Most important is during the trip into the Cave of Red Crystal to find a Psynard, where you'll see her drop her ID. Grab the ID, then visit her office in the Centropolis, on the third floor of City Hall. You can either invite her into the party or tell her to leave you alone.
Chisato is an excellent final party member, if you can wait this long to hit the full eight. Speedy and highly acrobatic, she has a powerful set of moves that strike many, many times in succession, quickly draining enemy health. Team her up with two other frontline fighters and you'll kill most foes in no time flat.