Imoen sucks, she dosen't advance in thief levels and she has no chance of getting as much XP as your other characters unless you haven't completed any of the Chapter 2 and 3 side-quests, Windspear Hills, Umar Hills, Cult of the Eyeless and such, I recommend that you keep your V'een, the only good NPC thief in the game is Jan btw.
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Worshipper of Tiax.
Wielder of the ancient Lawyer spells.
Member of the HADB clan.
Ally of the Fluffy Queen and the Dreaded Red Fluffy
General of the highest class and Leader of the Legal Department and Jumprope Compound in the IW Peacekeeping Force.
I've also got a pretty long sig.