It's always been clear to anyone who know's Spike's history with relationships, he has a very specific type... and Buffy, well, she's just not it. One shot. Spike X Dawn! Kind of fluffy at the end.

Spike's first name wasn't really William until after he died. He did use the name once while living, when trying to woo Lydia Bennet. Crossover!

Dawn remembers another William and allows herself to grieve. Later, she discovers something unexpected in a Watcher's journal.