Who is she? There isn’t really a lot of information to be had, and apparently that’s how the Gaimans prefer it. Fair enough, and thanks to Mary Warner of the blog Woo Woo Teacup Journal for gathering what was out there. She posted her findings here, and basically they’re a series of links to various scant mentions of his wife by Mr. Gaiman. The first link is to an online journal entry in which Gaiman says this: “my wife is happier to be a shadowy and mysterious figure in the background, or something.”
For the record, Mrs. Gaiman’s name is Mary T. McGrath, she’s American, and the couple got married before Gaiman hit it big. They have a son and two daughters, with one of the daughters still pretty much a kid and the other children both grown up and pursuing careers (Google for the son, film production in Hollywood for the daughter).