User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5247339-20130801122856/@comment-3146930-20130801161718

It isn't that they are ghosts, but you have to look at how the dead are portrayed in the series. When a person dies, their soul does go to the Underworld, but souls in the Underworld aren't really ghosts. They still have physical forms (depending on the situation, I mean some people have forms and others are more like ghosts). The best way to describe them is incomplete humans. Besides Phineas, who died from drinking a deadly poison, we haven't seen any undead actually die so it is possible that they simply reform in the Underworld and then exit using the Doors of Death.