Seriously? Where is this? Even if it's implausible, it'd still be a good read
I'd love to read this as well. With a another semi-guaranteed sequel/spinoff after P4A, there's still time to retcon this in.
I think I may have read it on the Atlus US forums. I did some scanning but since I came across it when Persona 4 originally came out, I haven't been able to find the original discussion.
That said, I came across a condensed version on TvTropes (of all places).
"How about Teddie being a shadow born from Nanako
Specifically he was born from Nanako's feelings of loneliness. Nanako may have been young, but as seen by her and Dojima's Social Links she has more then enough problems already to have a Shadow. She is trying to come to grips about death, misses her mother, and her father is almost never around. I don't think I recall seeing her spend time with friends either, she just goes home and watches TV.
Now look at Teddie. He's a shadow who wanted to be accepted by people. Nanako's loneliness could translate into Teddie's desire for friends. Teddie also very quickly hits it off with the Main Character, who is also the first person Nanako is able to draw close to. Teddie also has a tendency to say things as though it were a commercial which is easily explainable by how much time Nanako spends watching TV, and even somewhat paralleled by her singing along with commercial jingles. When Teddie becomes independent enough to form his own self and enters the real world, he very quickly becomes a friend to Nanako, who may in fact be his best friend besides the Main Character.
This would also explain the absence of Nanako's Shadow when she was in the TV world. It wasn't because she was too young to create a shadow, it was because her Shadow was already there. Furthermore it's Nanako's "death" that causes Teddie to return involuntarily to the subconscious and have his existential crisis, along with having his epiphany about just what he was."
There's some discussion about the implications of this theory:
"While this makes a certain amount of sense, it means that Nanako is going to be a raging lesbian when she grows up."
"Er, well be that as it may, it does nothing to discount the hypothesis itself, it merely point's out an additional consequence of it being correct that This Troper hadn't thought of while typing it. Besides, it's not like possible homosexuality is anything new to Persona 4 anyway."
"Perhaps not. For all of Teddie's "scoring with girls" obsession that winds up consuming his whole Personality it seemed to be entirely imprinted upon at first. When you meet him he's not like that at all in fact aside from being childlike and friendly doesn't seem to have much of a personality. It's not until his encounters with Yukiko' shadow that he starts to talk about scoring all the time, and the fact that the next two dungeons are also related to sexual themes just reinforces the notion. This doesn't make him a perverted bear by nature, just extremely impressionable like a child. Unfortunately the people he imprinted upon happened to be a crowd of hormonal teenagers."
I remember there being more to the theory but I don't even remember the game very well to recall all the different points the original author used. It did seem overly complicated, though, so I have zero doubt that it was never the intention of the original designers.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/Persona4