Rather than Shadows becoming aggressive when the fog lifts, they become aggressive on a Full Moon. Due to Teddie being affected by this, he knows he’s a Shadow from the start, but still has his identity issues due to wanting to know how and why he was able to make himself so different from a typical Shadow, and later, why he was able to leave the TV when he doesn't have a Persona.
Teddie ends up killing anyone who comes into the TV World on a Full Moon after they’re trapped by their Shadow, as his want to remove anyone who is tossed in is twisted in his feral form, causing him to attack. He’s able to suppress the aggression though he has a harder and harder time doing so as the Full Moon approaches, causing him to be snappy and impatient with the group, and depending on how close it is to the deadline, he makes violent threats against them.
He refuses to let them enter at all on a Full Moon, whether it be during day or night, as he can’t suppress his violent urges, and is worried that he’d kill anyone who comes in.
Teddie decides to create a human body in order to blend in, and to try and leave the TV World behind, thinking it could potentially be a way to stop him from being affected by the Full Moon (he's capable of doing so because the rules regarding Shadows is a little different here, of course - Teddie being able to develop his human self is a extent of his already existent ability to make himself different from other Shadows. Him being able to leave the TV World after getting a human body is due to simply being so far from what a Shadow is that he isn't bound by their rules regarding leaving and entering the TV World).
Unfortunately, it failed, and he’s still affected. Luckily, he managed to leave Yosuke’s house before he was fully gone (Yosuke still took him in despite Teddie’s repeated attempts to stop him), though he ended up killing a security guard who caught him trespassing in Junes (as he was trying to go back to the TV World in order to prevent himself from attacking people).
He also killed Mayumi and Saki, and is very guilt ridden about it, but the others promise that they don’t hold it against him, and whoever threw them in the TV will be found and will answer for the crimes and using him as a tool for murder.
Rather than getting a Persona from defeating his Shadow, it fused with him, though it wasn’t apparent or noticeable until the first Full Moon after he created his human body, which he made prior to the deadline to rescue Rise. Here, his human body and bearsuit fused together, giving him a permanent mash-up of the two. Due to his bearsuit also being fused with his Shadow, his eyes are missing, and he has a big hole in his face, though he usually has the hole bandaged up.
His body is hollow, with a zipper on the neck, meaning his head is easily popped off without much issue. He’s also filled to the chest with a thick, black sludge, essentially the material Shadows are made out of. Despite being fused with his Shadow, it has no bearing (haha) on his ability to enter and exit the TV World.
Due to being fused with his Shadow instead of gaining a Persona from him, Teddie gains his Shadow’s moveset, capable of doing things like Nihil Hand. This also causes him to have spirals of Nihilism, but he tends to keep himself alone until it passes.
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