Velna is one of the enemy Mystics that is seen throughout the game. She is quite mysterious during the first scene with her but later reveals how evil she really is.
Velna is one of the strongest enemy Mystics you'll encounter, excluding Avalon. She uses her own Origin as a defense to survive instead of an offense for the most part. Of course, Vestra (her Origin) is used to attack the party with special attacks. But without Vestra, Velna is quite weak.