http://www.gameinformer.com/b/features/archive/2015/08/10/destiny-the-taken-king-39-s-new-approach-to-storytelling.aspx
6:15 into the vid, Luke Smith confirms that Eris Morn is a Hunter.
And you thought a Warlock could survive in the pits of the hellmouth. Nah, only a Hunter has that skill.
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I'm glad. Warlocks are known for being intelligent, whereas hunters as devious and treacherous. She abandoned her squad, leaving them to die, and sided with the enemy.