As is the case with many members of The Old Guard, little that is concrete is known about the late Doctor Lucifer Mongfish, apart from his relationship to the current protagonist; in this case, he is Agatha's maternal grandfather. However, before Lucrezia Mongfish apparently converted to the cause of Good and married Bill Heterodyne, we are assured that there are many stories of The Heterodyne Boys having occasion to foil the Evil Dr. Mongfish's plans, and those of his daughter.
Dr. Mongfish is first presented in the apocryphal Heterodyne yarn, The Dragon From Mars, as a revenant slave to the Dragon. Once he is restored to normal, he doesn't show many evil tendencies as he works along with the Heterodyne boys to thwart the Dragon. His Good traits in the story are consistent with his description (no longer available) as having reformed after Bill married Lucrezia.
Possibly relevant outside informationEdit
The literal meaning of the word "fishmonger" is "a man who sells fish". However, in Shakespeare's Hamlet, Hamlet says he recognizes Polonius — the prime minister and the father of Ophelia — as a fishmonger. (Ophelia has been Hamlet's sometime romantic interest and he is unhappy with the way Polonius is influencing her.) The usual (though not universal) interpretation of "fishmonger" in this case is as a euphemism for whoremonger.