It's Thessaly. As Radhil says in the comments, we know from various clues that Thessaly is the girlfriend they're talking about. In A Game of You, Morpheus and Thessaly meet, and Thessaly shows some interest (although she seems more annoyed: "He comes on like he's so cool. Who does he think he's fooling. Well, he's not fooling me. Oh no . . . and it isn't even as if he's good-looking. He's too thin for a start."). And then she tells the story about the relationship and breakup in The Wake.
But there's a particular link with the pendant. At the end of World's End (Sandman volume 8), Brant is telling his story to a bartender who looks like Thessaly, and she seems to be wearing the pendant.