They are:
1. Astaroth: The King of Earth, known for his cruel and manipulative nature. He is also the one who implanted the demon seeds into humans, leading to the creation of exorcists.
2. Lucifer: The King of Pride, considered the strongest demon king. He is the twin brother of the main protagonist, Rin Okumura.
3. Beelzebub: The King of Gluttony, known for his immense size and appetite. He is also one of Rin's close friends.
4. Leviathan: The King of Envy, known for his immense size and ruthlessness. He is a master of water manipulation.
5. Mammon: The King of Greed, known for his love of money and material possessions. He is also the most cunning of the demon kings.
6. Asmodeus: The King of Lust, known for his beauty and manipulative nature. He is also a skilled fighter.
7. Belphegor: The King of Sloth, known for his lazy and carefree attitude. He is also a skilled inventor.
There is no mention of an eighth demon king in the series.