Who are Greeds?

On the top pic are the Greeds, Kamen Rider OOO's main villains. From your left, we have: Gameru, Cazari, Mezuru and Uva and as we all know, there is one Greed is missing in the group, namely Ankh who would later give the OOO Driver to Hino Eiji.

All of the Greeds are currently incomplete because of the medals were sealed in various places. They also have mysterious belts.

They have two kinds of medals. The one with colors is the Core Medal, it's the core of a Greed's existence. Medals that are silver are known as Cell Medals. These make up their bodies. The Core Medals are like their hearts, souls etc. While Cell Medals are basically like cells in a living thing.

Ankh is a Greed with not that many medals, which caused him to become an arm. This is why he took the body of the policeman. Like all the other Greeds, he's collecting medals. To make up for his lack of power, he uses Eiji. What's different about Ankh is that he is sly and sharp sighted unlike the other Greeds.

When they put a Cell Medal inside a human, a monster called Yummy is created. Their source of power is desire. How the Yummies are, depends on the Greed. For example Uva's main desire for Yummies are money, wealth etc.

Source: Picture - alternate-zer0, Information - DecaEnd