That said, you think D.Gowther is a 10C?
possibly D.Gowther existed even when he was mirroring O.Gowther, but at the start it was only an empty doll, which learned about how to act as O.Gowther from all the time he was used to mirror O.Gowther
like what happens with machine learning, D.Gowther was being guided, and in the process he also learned from O.Gowther, so progressively he became like the original one
the line in becoming a person should have been when the D.Gowther had learned enough from O.Gowther to no longer be only a doll
the line instead about him being his own person, i think it should have been once he had to start acting on his own intention, without any guiding, so most likely once O.Gowther died
O.Gowther himself said he may be a 10C but not loyal to Demons and doesn't care about holy war.
I think truth is D.Gowther is not.
Wonder how much of this Med knows