Vritra

Vritra is one of the Five Great Dragon Kings. He is the Evil Black Dragon known as the Prison Dragon.

Appearance
A black Eastern Dragon.

Personality
Despite being called the Evil Dragon, Vritra does not show any of his evil personality. In fact, he appears have a wise and very polite attitude. Whether or not this has anything to do with his soul being split into several pieces remains to be seen.

History
Prior to the series, he was vanquished by Indra and his soul was split and sealed into multiple Sacred Gears, one of which would end up in Genshirou Saji. There appears to be more than four Vritra Sacred Gears which are held by others, but it is unclear if they also hold parts of his soul (like the evil part).

Birth of the Breast Dragon Emperor
Vritra was mentioned in Volume 4 when Ddraig explains to Issei about Ophis and the Five Great Dragon Kings.

The Heroic Oppai Dragon
In Volume 7, Shemhazai, under Azazel's orders, fused all of Vritra's remaining Sacred Gears into Saji, restoring Vritra's conscious.

In Volume 9, during the battle against the Hero Faction and Yasaka, Vritra having regained consciousness, was shown to have taken up the role as a mentor to Saji, instructing him on how to use his powers like with Ddraig and Issei.

The Legend of Oppai Dragon and his Lively Companions
In Volume 14, Vritra along with Fafnir, help wake Ddraig up from his sleep during the attack from Euclid Lucifuge and Grendel from Khaos Brigade.

Powers & Abilities
According to Azazel, Vritra is the weakest Dragon amongst the Five Great Dragon Kings in terms of raw power. However, his wide variety of techniques make up for his lack of power with versatility.

Trivia

 * In Hindu mythology, Vritra was an enemy of the god Indra.
 * Vrita is the only Dragon King who is also an 'Evil' Dragon.