Souls

There are two type of Souls - Plus and Hollow. Plus is the most common one that roams around places and does no harm, but those with strong regrets in the current world will bind themselves to their focus of regret by the Chain of Fate. If the focus is a person, they become 'obsessed spirits', and if the focus is a place, they become 'earthbound spirits'. Hollow, on the other hand attacks other beings (mainly those with high-spiritual density) and eat their souls.
 

Earth-bound spirit

Those who are naturally assimilated with the land. They will only show itself when someone trespasses their territory.
 

Grand Fisher

A name given to an infamous hollow who hides his body from being seen while his neck protrudes a lure that takes on a human shape, and devours souls with high spiritual density to increase his own strength.
 

Hollow

All hollows are formerly human's souls or Plusses. There are many reasons why the Plusses become Hollows - they're not brought to the Soul Society by a Shinigami, escaped from Soul Society, not protected by other Hollows, or when they wanted to fill their empty heart and seek out their loved ones in their past life. Those who has become hollows eat souls to ease their pain and suffering, and cannot revert back to a normal soul.
 

Menos Grande (Arankal)

• Gillian - Gigantic hollow that is born from hundreds of hollows that pile and mix.
• Ajuukaru - Much smarter and stronger than a gillian, like a vice-captain compared to a seated officer. Not many of them exist or so it is thought!.
• Vastoorode - Looks like a shinigami and is much stronger than your average captain. Not many are known to exist the few that do exist are only seen in Hueco Mundo. It is said that 10 of them would bring destruction to the Soul Society.

Plus

Plusses are the soul that has been separated from the original body and would not harm any other creatures. However, they have the potential to become a Hollow because of several reasons, whether they are willing or not.
 

'Target' Leech

Not a hollow, but it's actually part of the main hollow called Shrieker. It does have a little willpower, and does not feel any pain at all.