So eventually I remember that he's the Scourge, and when I search for that on Wikipedia the sixth and eight entries that come up are respectively Hero (the Angel episode with a gang of maraudering demons called the Scourge) and The Scourge of the Underworld, the guy(s) from Marvel comics. And when I read the latter entry and found the origins of the SotU I discovered that it's actually an organisation "financed by the original Angel, a hero from the Golden Age of Comic Books"
Coincidence? I think not... Nice one, Joss.
Now I'm horribly tempted to bring the SotU into the story, since they turn out to be based in California and that's where Nina comes from, but I think I shall have to resist the temptation, the plot I have in mind is complicated enough as it is.