+Both Aquired a taste for killing
+ Both are neck aficionadoes ( he snaps and sucks and she chokes)
+ Both lurk in the night
+Both went bad after having sex with one of the Scooby gang.
+Both have a leather pants fetish
+Both have a sadistic side
+Both end up in chains, in the mansion
+Both have just one name and no known last one.(Oz could work here too)
+Both names stemed from religion
+Both have song titles ( both songs about messed up people)
+Both going to leave the show
+Both buffy's lost causes she stands up for
+Both spend a long hours at school but don't actually attend.
+your either love or hate em.
If you can think of more, email me!