Imoen is a pretty rubbish Thief who never will get past level 7, but, like other people have said, potions and spells and items will enhance her abilities considerably.
You are right about Yoshimo. Best Thief in the game!
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SPOILER ALERT
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After it was way too late for me, I was given info on this board on how to keep Yoshimo. If you want it, post. If I'm not here to tell you, someone will
F.