Vritra

Vritra is one of the Five Dragon Kings. He is the evil Black Dragon knwon as the Prison Dragon.

Appearance
A black Western Dragon.

Personality
Despite being called the Evil Dragon, Vritra does not show any evil personality. In fact, he appears to be a wise and quite polite Dragon.

History
Prior to the series, he was vanquished and his soul was split and sealed into four different Sacred Gears, one of which would end up in Genshiro 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).

Plot
In Volume 7, Shemhazai fused all of Vritra's remaining Sacred Gear into Saji, restoring Vritra's conscious.

Powers & Abilities
Though its power might be the lowest amongst the Dragon Kings, Vritra is the king of techniques in terms of variety and unconventionality.