Buffy and Spike are coworkers and complete opposites. She's never left the state he's traveled the world. They butt heads from time to time....it could be nothing major or they could hate one another. 

Christmas is coming up and Buffy has caught Spike in a lie. She's heard him tell different stories to different people on what he'll be doing for the holiday. 

Buffy calls him out on it but is shocked when Spike grudgingly tells her the truth. Ever since his mom has died Spike has no family left nor no one to spend Christmas with. He was just going to sit in his apartment and watch Netflix. 

No matter how much he gets on her nerves Buffy can not bear to let that happen. So she invites Spike to come home with her for Christmas. 

Ideas I had was that Giles is her stepfather, more like dad. Who is Dawn's dad and she's Buffy's half sister. 

Spike wins over Dawn and Joyce at the start. Giles takes some time. ....over protective dad.