Well there's two ways to approach this: mechanically or thematically
Yoshimo specialized in setting traps and snares as a theif with the Bounty Hunter kit. Trapsmith is probably the closest you are going to be able to find, even though arcane casting on him makes no sense.
For Nalia Rogue 1 or 2 into Wizard X is your best bet. Mechanically, she starts to become a terrible thief almost immediately, but ends up being one of the best arcane casters in the game, second only to Edwin.
Don't forget that Imoen, like the player character was a Bhaalspawn, and both of them gained a number of unique SLA's depending on their alignment (Vampiric Touch, Magic Circle, Curse, Cause Fear, and the like... man I can't remember any of the good-aligned ones
). And the PC had the ability to turn into the Slayer. They never really describe what it looks like in the novels (they say "human-sized but bestial"), but they use the model for a demon (a miniature Balor I think) in the acutal game.
Haer'dalis was a tiefling, and a Blade (class kit for a bard). He gains potent combat abilities in offensive spin and defensive spin but gives up some of his spellcasting and the effectiveness of his his bard song. It's probably easiest to just stat him as a dervish, but bard could also be made to work with snowflake wardance, slipper of battledancing and the like.
HOWEVER... personality-wise he was a Doomguard, that is, a person who believes that the descent of all existence into entropic annihilation is not only inevitable, but a good thing. It's possible that there is a presitge class for that faction printed in some obscure Planescape book that might fit him better than bard or dervish.
Do you already have plans for the antagonists too? (Irenicus, Bodhi, Aran Linvail, Saemon Havarian, etc)