There is no discussion. Eris Morn is a Warlock. Confirmed by Bungie.
Here is the interview:
https://gotgame.com/2014/12/19/destiny-interview-bungie-community-manager-eric-osborne/
Here is the quote:
Eric (Osborne) – Yeah, she’s a Warlock, so she’s a Guardian. She’s a protector of the City.
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Ok then