It's "Choir Boy" in my mind. It just happened when I was writing and now I can't shake it.
I do like Gala's "Fluffy" too, tho. 
I don't think Varric will be repeating himself.
Besides, Sebastian led the Chant in Kirkwall sometimes, and hence the choir. I don't think Cullen will be doing that, I'm sure Inquisition has real priests, who are less busy with trying to keep our army from murdering and looting on the territory we are supposed to be protecting 
Oh how I will laugh if Varric won't be able to come up with a nickname, because Cullen will have an Aveline effect on him
But in my opinion Cullen is only scary when he wears the helmet, so maybe it will be the Knight-Captain, because Varric remembers him from Kirkwall like that.
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