Well to me the whole thing works best if Carlos was just Carlos.
Why?
Simple - we've known Harry is treated as suspicious or probably-evil since day 1 of the series, but what we see is him consistently proving to e.g. the Wardens that he's really not as bad as they suspect. As powerful maybe, but not as bad. So how'd he ever get to be mistrusted?
Enter Carlos - a notionally upstanding Warden who used to like Harry. Who probably still likes Harry as a person. But because Harry is canoodling with Mab and Lara, and constantly threatening Carlos with Molly (even if Harry doesn't know it), he gets damned suspicious. Add a bit of crazy in the fight with the vampires that killed his other friends, and suddenly that Harry-liker is one more of his enemies - and Harry'll have an uphill struggle if he wants to be friends again.
In short: Carlos-as-human is there to explain/remind why good upstanding people can be Harry's enemy.