Ya know, I always sort of thought, in so much as always covers the few minutes I've entertained the question, that Spike was something like a monk.
I mean, he doesn't seem that attached to equipment, he's pretty generally athletic, and he does have a good variety of useful skills.
He does grapple a lot and seems to have a plethora of intriguing special abilities that don't truly seem to come from anywhere.
Most of his dramatic tension comes from his conflicts with various duties and strictures. You just replace the general ki style lawfullness with warped ideas of love or violent evil or pretty unforgiving good.
You can put a few ranks into perform with monk.