Not that I have anything against Imoen, Minsc, Jahiera, or the other great NPCs in the game, but I was wondering if having any one of these characters in the party is necessary to the completion of the game (or the availability/completion of certain quests)? I happen to favor the characters I created in BG/TOSC and want to continue using them ALL in BG2. If anyone does know, please tell me without giving too much away--I'm taking my sweet time with BG2 (on purpose).
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Less talk, more punch!

I'm only trying the game for the first time with all my own characters now but i'm sure you can finish it alright this way. You do miss out on quests though as the different npcs have several special quests during the game as their past comes back to haunt them. I found you also don't get all the quests if you pick up characters late in the game for instance. You also don't get any interaction between the characters, romances etc at least as far as I've seen, and i'm up to chapter three. Of couse you wouldn't get them complaining about your actions either!

You need to use Imoen, but I suggest you don't use your chars from BG1. You should play the single-player version first or you will miss A LOT of quests, cool dialogue, and fun.

Imoen??
You do not need to use her. I finished the game solo and not once did I have to use imoen.

Just out of curiosity, is the story still the same as far as her getting captured for using magic by the cowled wizards?

Yes the story stays the same. she gets captured after you leave Irenicus' dungeon whether you have her in your party or not.