In Buffy the Vampire Slayer, Halfrek is a vengeance demon who, like Anyanka, was the property of D'Hoffryn.
In the Season 6 episode "Older and Far Away", Spike and Halfrek find themselves in Buffy's house at the same time. Halfrek calls Spike by his human name, William. The Scooby Gang are surprised at this, but Spike and Halfrek refuse to comment.
Halfrek is played by the same actress (Kali Rocha) who played Cecily, the object of William's affection when he was still human in England, and so that seems to be their connection, i.e. it is implied that Cecily and Halfrek are one and the same. (This was actually confirmed in the commentary for the Season 7 episode "Selfless" — thanks @Richard for pointing this out!)
Question: Is there an explanation (e.g. by Joss Whedon himself or in the comic books) as to why Spike did not query Halfrek further about her life since their time in 19th Century England?
Given Spike's underlying sentimental nature (even in the face of repeated heartbreak), it seems out of character for him not to take a more direct interest in what has become of Cecily.