Jack's dead wife. In life she was a loving wife and mother, starting a new career as a journalist, which was all ended when her car was struck by pickup at an intersection on her way home from work. The drunk driver survived the accident, even getting off with two years probation and community service. Jack's "family" ensured justice was served to the man.
Not even Megan knows exactly why she was pulled, partially, back from the dead into the physical plan after 14 years, but while she's here she's got almost a decade and a half of mothering to catch up on.