When someone who dies still linger to life and there is some kind of regret, the soul remains on the world as a residual shaped existence, with their principal component being only ether. It is difficult to identify someone as a ghost due to their appearance being the same as they when they were alive. Their only distinction is that some of their body parts are blurred (mainly the legs). Physical attacks don't work on them, as the blow goes through the ether, but they are able to carry and stand on physical objects. They can eat victuals but require to be consecrated before.
According to Toshiie Maeda, there are some conditions that are essential to become a ghost. It is known that one of these is to feel negative emotions and a deep Lingering affection for this world (この世に対する未練, Konoyo ni taisuru miren) at the time of death. If said regrets dissapear, the ghost will meet final death and ascend leaving the material world behind. At the very least, it can be said that to keep existing after death is very difficult to achieve, even if is noted that the ones who have a Inherited Name tend to last longer.
There are three main types of ghost, the Floating-type (浮遊型, Fuyū-gata), Suicide-type (自爆型, Jibaku-gata) and the ghosts those who wished to be like this, the Reassigned-types (転属型, Tengoku-gata).