The NPC's that you take first and keep until the end of the game will get the most experience from all the quests and the killing you do. Taking an NPC with a specific quest will give you experience but it only matters to the others if you get rid of this NPC again. This means that the best option is to take 5 characters and exchange the sixth from time to time for all the experience. This only means that the last character joined will be the weakest.

possible spoilers ahead

Imoen will be as weak as when she left your group. I wanted to get Korgan in chapter 6 but he was still only level 12. He didn't do anything while I did so much insane adventures. Just drinking at the Copper Corronet (or just standing there). Too much of an investment needed if you ask me. I would rather go on with everybody else.

Even when you get the chance to summon old party members in the pocket plane, everybody is of much lower level that all the experienced members. Or is it just that I am too afraid to lose some experience and start all over again with a character and his abilities? Too afraid to lose my great party. I have seen this in other (power) gamers as well. Didn't know I was one, though.

Well, I do the same thing too, but part of it for me is also loyalty. I figure my character has been through quite a bit with this group, and it would take something pretty big to separate us. This also makes me very choosy about who I let into the party, to begin with. The one exception was when I ditched Valygar for Sarevok. But Valygar was really only let in as hired muscle, because Minsc gt splattered...

I did the same thing with Valygar and Sarevok, although my Minsc was still alive. Never told Valygar he was just hired. I was just being selfish taking Sarevok. And it wasn't the family thing (something Valygar wouldn't believe either) but Sarevok was just a better fighter. I had to save the planet.

sarevok > valygar. minsc > sarevok...atleast in the game i played. during the last battle, minsc had -11/-11 THAC0. he was lvl 25ish i believe. axe of the unyielding, and that +5 sword only good chars could use.

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~ One by one, the penguins are stealing my sanity ~