Ghost

"'Ghosts!'"

-Anne Brontë-
Ghosts are creatures found very often in the series. A ghost is a deceased person who has been/is seen in the living realm. Not every deceased person is able to be summoned.

Ghosts in general
Ghosts can't come back to the living world out of themselves, the first exeption in history is H.G. Wells who (in the "further place") made his timemachine complete after many hundreds of years and traveled back to a month after his death to join his love.

In any other case, ghosts need a seance to be summoned or even brought back.

Summoning to ethereal form
If a ghost has unfinished business and is "down to earth" according to Krishanti, a good ghost summoner can bring them back in ethereal vorm. Lenore still had to say "I love you" to her fiancé Guy de Vere and so, famous Krishanti Ravenwulf was able to bring her back.

Krishanti tried to bring Guy back but he ended his life out of free choice so she couldn't do it. Also, all his business was finished when his true love died.

Annabel Lee still needed to confess her love for Edgar when she died, so she was able to be brought back in ethereal form. It is unknown by whom.

Summoning
If a spirit has something to say, he or she can be summoned by a summoner for a couple of seconds. But the spirit will still be transparant and talk vaguely. Fyodor, Mary, George and Guy were able to be summoned by Krishanti because they had information about the murderer.

Time-travel
When H.G. Wells died, he hadn't confessed love to Lenore but Lenore didn't want to bring him back because she didn't think he would want to spend eternity with her. But in death ("in Heaven or whatever") H.G. Wells has all the equipments and it takes hundreds of years but he returns to Lenore...around a month after he died.