William Piper was a mortal with whom Gwen fell in love, and was the father of Marnie, Dylan, and Sophie. Gwen subsequently moved to the mortal world and stopped using magic, which upset and possibly angered Aggie, and it is implied that by the time of the movies he has died. He is only seen in Halloweentown in a family photo taken on his last birthday.
He and Gwen met on Halloween night, sometime before Marnie was born, when Gwen decided to crash a Halloween party that he was attending. They married afterwards and Gwen moved to the Mortal World to be with him.
It is assumed that he is dead, as he is never seen in the series, and is mentioned in the first movie as having passed away.
Presumably he met Gwen 14 to 15 before the film, as Marine is 13.
William likely passed away from a disease, as many other Disney films and shows have their characters noticeably saddened if a loved passed away from an accident.