I've been mulling over this one for awhile, and now that i've gone through another theory involving him, i find it to be worthy of posting. Just a little something i thought up while wondering about the still-unknown past of good ol' Kugo.
As far as i've learned through some research, this isn't quite a new theory. Still, i've decided to share.
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I'm wondering if Ginjo was maybe the result of a Hollow experiment performed by Aizen. We all know Aizen did Hollowfication experiments on Shinigami, but is it reasonable to speculate that he may have done some mild experiments on the side to see what would happen to humans who were exposed to Hollows? Not necessarily Hollowfication, per se; but experiments involving Hollows, nonetheless. (Or, in actuality, they didn't even need to be experiments at all. Maybe just observation and study of normal Hollow behavior. Aizen was deeply dedicated in his research of Hollows, after all.)
This could explain how Ginjo became involved with the Gotei 13. Contacted by Aizen, perhaps?
Aizen monitoring the side effects to Hollow exposure, he observes Ginjo as being born of a woman who was among the victims. After some time, he notices Ginjo's enhanced Reiatsu (and possibly unique power, if Ginjo had acknowledged it by then), and seeks to learn more about this new specimen.
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I have more in store to this theory, but for now i'll just drop the basic; i'll share more through discussion. So hopefully i can get a good thread going on here.