Ghosts are familiars under the Necromancy tree of magic, thusly astral beings. It can be summoned from the rune of Hades Gate, and serves magicians during battle by collecting mana from crystals. Necromancy seems to stand strong against Sorcery, since only "through the pleasures and pain of flesh that devils are able to tempt man", and as an astral which is "a strong will without a body", they have higher chances of winning against a familiar from the Sorcery runes. Also, they don't receive damage from physical attacks.
Other than attacks, they gain the ability named 'Ghost Charge' after the Hades Gate rune is leveled up to a certain degree, by launching a sacrificial attack which requires them to charge into their opponents.
Being able to float through floors and walls, Ghosts give the player an advantage in the initial stage of battle as they don't need to take the lengthy route to carry mana back to base like Elves, Blobs, or Imps.