What IS that avatar? Is it just a thief w/ special armor? if so, what? or is a 'Shadow thief only' avatar. I've tried changing yoshimo's colors to red and donning black armor, but to no avail. Is shadowkeeper my only hope?

It is a special avatar. You can't get it by any normal means, and it doesn't come with different armours. Use Shadowkeeper or CLUAConsole.

With the cheats enabled, you can use Ctrl-7 and Ctrl-6 to change your avatar, but you loose your paperdoll, and you can probably change your looks with a nice CheatLUA-code, but i don't know which....

You cannot use CLUA commands for avatar change.

You may create and use a custom script to POLYMORPH you (in fact your avatar) into anything that is contained in ANIMATE.IDS.

But as I see things, you'll use ShadowCheater and after a while you'll post here about some strange happenings with your game noone ever heard about.

That's why I use only ctrl-8 cheat. ... ok. CLUAcreateitem too, if I need something like golden pantaloons or other BG1 stuff.

IIRC changing avatar with cheatkey combo works only till you reload the game. The avatar you get by that means won't stick to your hero permanently.

Yup! Extremist is right once again. But don't mind me being ignorant: I don't know much about the CLUA. I did know about some options, but they have many restrictions...as Extremist has shown.
;)

The Console's primary purpose is testing/debugging.

However, as it always happens, Console's "powers" are easy to abuse, especially when you find out that ShadowCheater cannot meet your taste.

Extremist, your probably right about that ShadowKeeper leading to bugs in game, It seems like something I would do, but so far, I'm quite reluctant to use SK, I don't want to mess up my saved games.

CLUAConsole:EnableCheatKeys()
Ctrl+6, Ctrl+7
'Nuff said :p

If you change your avatar the game will crash if you do something that avatar hasn't got an animation for.

Thats is not really true, if you have a character looking like, say, a fat man the game will not crash if you tell him to attack tough he has no animatoin for that, the only problems are sleeping avatars or static ones that can't move from their position, like standing in cement too long, you get stuck

And if someone casts dispel magic on you you lose that avatar too.

Not really, some avatars can do things they don't have animations for. Like the peasants can attack. They just stand there and deal damage. Try attacking with a water weird though, that'll screw your game :p

not always, not when I tried