Concept stolen from Granddad, aka
notsouledyet, aka the great bloody git. Bits actively plagarized because, hey, why work harder than you have to, really?
Imagine for a minute that during a nebulous time somewhere around the end of season 2 of Buffy the Vampire Slayer and the beginning of season 3, Spike fell down a rabbit hole and ended up getting that chip in his head that prevents him from causing physical harm to humans. Imagine further that he fell down a second rabbit hole while escaping and, somewhat confused, has found himself on an island with a high school, lots of magic and saddled with a total inability to snack on the human population. This means that he is completely clueless about anything mid-season 3 and beyond of 'Buffy the Vampire Slayer' and anything 'Angel the Series' related.
This also means that Spike has no inclinations toward warm, fuzzy moments unless your name is Drusilla. Period. Said warm fuzzy moments tend to deal with death, distruction and really disturbing goings on anyway, but hey. His relationship with Angel/us is somewhat more complicated, particularly because he still doesn't know which one he's dealing with and isn't going to be dumb enough to ask if he can help it.
I am also stealing the idea put forth by Buffy's player and embraced by Angelus that there were weird glimpses of other places and people on his way here, so that he'll recognize some of the other people from later places in canon... but not necessarily have a good idea of who the hell they are.
He's very, very confused by Angel/us as high school student and Drusilla as psychology professor, but he's rapidly finding his feet.
I'm a very lazy person, so allow me to link you to the
wikipedia entry and then sum up.
William Pratt (because I'm going with Joss' official word on the matter and also because it's a family name of my housemate which amuses the living hell out of me) was born in 1850 and grew up to be a dreadful poet who lived with his mother. Rejected by the object of his affections, he ended up with Drusilla... and immediately became a hell of a lot more interesting.
Spike has killed two slayers, something of a record to beat, and got the nickname 'Spike' due to the imaginative use of railway spikes on various victims. If you cherished any images of tortured, emo Spike... it'll only happen if Drusilla or Angelus decide to deliberately torment him in that direction. He likes being an asshole. He enjoys Drusilla's madness. He subscribes to Angelus' idea of family in that if there is no pain or torture, you don't really love him anymore.
Public mocking and/or humiliation will bring out the absolute worst in him. He may not be able to attack you (if you're human, that is), but that doesn't mean he can't get someone else to drive you to suicide while he sits happily with the popcorn and watches. He's got a very definite 'I'm a small, spoiled child and I'm going to have my way and have it right now' mode. He's impatient, occasionally reckless, over-confident, arrogant and likes to leap straight into the pain and violence. This puts him at sharp odds with Angelus as Spike is definitely a 'shoot first ask questions later' sort of person when given the opportunity.
There are two people in these parts who Spike will actually listen to. Drusilla, who he'll not only listen to but basically dance rather ecstatically and disturbingly to whatever little tune she's humming today... and Angel/us. In the second case, there may need to be beatings in order to get him to actually acknowledge what was said and force an agreement out of him... but he will, at least, listen. If only to mock. His time with Darla, Angelus and Drusilla was quite possibly the most enjoyable time of his entire life - which should tell you a lot about how screwed up he is right there.
Spike's running a pub because, well, he's stuck on a damn island with no money and, let's face it, Spike is a total attention whore. He's in it for the tips, for the flirting and for something to do of an evening because this place is just too damn small for anything really fun not to come back to haunt him. He encourages teenagers, because there are lots of them, but tries to keep things for that subgroup as non-alcoholic as possible because there's nothing more annoying than a stupid drunk teenager when you can't kill them for pissing you off by being, well, stupid, drunk teenagers.
Spike is an easy mark sometimes because he does tend to react emotionally instead of logically once you've pushed his buttons. He's not stupid, however, so if you keep pushing the same buttons, he'll find more amusing ways of reacting - probably to the detriment of whoever is pissing him off.
I've always seen Drusilla's deserting Spike as the straw that broke the camel's back and led to every idiotic, self-destructive decision from then on. Every relationship, every choice... if Drusilla hadn't kicked him in the head, he probably would have gone on more or less as he was. As it stands, I think he could certainly handle Drusilla taking up with someone else - albiet spending every spare moment plotting their horrible demise - as long as she didn't just leave him.
Angelus he sort of has to live with, and we've all seen how happy that makes him. Really.
Here, Spike still has his dark goddess... and hence he'll not be displaying any of the stranger, more self-destructive behaviors that he developed in the series. Well, not unless she runs off with the giant squid or something similar.
Last but not least, take this to heart. Spike is evil. Drusilla sired him and he is totally devoted to her. He's insanely jealous of her relationship with Angelus... and anyone else she looks sideways at. He's a nasty, psychopathic, blood-drinking, sadistic murderer and will be happy to attempt to screw you over if you mess with him - and not in that fun, third date kind of way, either(1). Just because he can't lay hands on you doesn't mean he can't find ways of making you wish you were dead.
(1) Credit to the evil one for the line.