Quests I've seen associated with characters. Some of these you can do without the character but you find out about them through the character and generally the character adds a lot during it.
Jahiera: Curse
Aerie: Circus, Playhouse
Anomen: Sister, Order
Cernd: Druid, Family
Nalia: Keep, Isaea
Kelgorm: Family
Korgan: Book
Edwin: Scrolls
Valyar: Planar Sphere
Jaheira also triggers some mess with the Harpers in the Docks Area.
I don't know for sure, but I think Viconia will trigger one or more quests in the Drow City. Remember those drow prisoners?? They belonged to House de Vir. And what's Viconia's full name? Right, Viconia de Vir...