Octarus

Octarus was an antagonist in the Buffy two-part episode "What's My Line" - he was the first of the assassins sent to kill Buffy Summers during the events of the story and was the first to die, he was notable for being one of the few Buffy villains who was not a demon, vampire or monster: this arguably makes Octarus even more evil since he had a choice to pick good or evil but choose the latter.

History
Octarus is a formidable assassin from the Order of Tarakas and was prophesied by Drusilla in the form of a one-eyed demon on a Tarot Card set, however when he revealed himself Octarus appeared to be a human who had lost an eye in some confrontation: despite this he was still very dangerous and ambushed Buffy while she was ice skating.

However Angel arrived and attack Octarus, surprisingly even Angel (a powerful vampire) was finding it hard to keep up with Octarus and Buffy acts quickly, using the blade of her ice-skate to slit Octarus throat and end his life before he could do any more damage.

Buffy herself commented on Octarus and mentioned to Giles that he was "hard core" - since she was the Slayer and Chosen One this line suggests Octarus was definitely no pushover despite his limited screen-time.