For instance, Spike = leather, tobacco, and blood. De Sade is a leather scent, Hellfire combines the leather and tobacco, and there's a number of variations on Dragon's Blood - scents.
(Note: chances are these would actually smell terrible together unless I just got really lucky with my combinations. But you get the idea.)
Buffy = sunshine and vanilla
Xander = ... I've seen chocolate, but I don't know if that's really entered fanon or me just being oversensitive to any reference due to my own association there. ETA: wood chips! Sawdust! Whee!
Outside BtVS-verse, we have Scully - the concentrated scent of strawberry shampoo. (Plus of course Willow tastes like strawberries....)
Who else? (eta - I was thinking of scents that have been over-associated with the characters in fanfic, more than scents that might actually be a good real or emotional representation of them - but that'd be a fun line to make up too.)