[The Hairy Caveman]

If you're going to make a thief NPC, not a bad idea considering their useful abilities and the fact that you only have 1 pure class option, you need to figure out what kind of personality, abilities, dialogue, ect. you want to give him/her before you start actually building it to save yourselves time. You should know, however, that Valen and someother assasin (she has a short short called cryblade, I forget her name) are out there. They are both evil so I would suggest either another neutral or the first good aligned thief. How about instead of the traditional burgler or backstabber you make a character whos more espionage oriented. He/she could be a spy sent by Belt from Baldur's Gate to keep an eye out on charname after the mysterious departure from the city. I don't know when to suggest his/hers first appearance but I DO think that the new spy should contact you, instead of the other way around, like every other npc in the game, after all the spy's mission is to keep you close. Could make sense to have contact after reaching spellhold or any other natural chokepoint in the game. It would also be great to see some of the elements of the infinity engines forgotten child (planescape)implemented. Would it be possible to initiate conversation with an in party character? Could be useful. as for abilities here are my humble suggestions

str 12
dex 16
con 14
int 17
wis 12
cha 18

Daggers, long swords and crossbows

just to make him more of a mole type agent (reflected in int and cha) As for personality I would make him a very "in the know type of character" who often knows or suspects the path the group must take before it is upon them. He should also be intuitive, able to tell when people are lying or hiding the true motives of their actions. And with him being from the city of baldurs gate it would be very easy to implement some kind of side quest...