Because they’re adorable as all hell.

What you need to understand about Teddie is, that he’s as clingy as he is because it’s in his nature to crave affection; that’s the very desire he is composed of entirely. Telling him not to seek out other’s affection is like telling a human not to want to drink water: He’s physically incapable of it. However, as much as the other members of the Investigation Team love him, they’re all awkward teenagers and often unable to wholeheartedly show how much they like him, because it isn’t in him, leaving him insecure and desperate.

When Labrys is around, that changes, because Labrys isn’t like them.

She’s outspoken, always saying exactly what’s on her mind, and when she appreciates Teddie, she does it wholeheartedly, petting him and admiring his good traits. This is really healthy for Teddie, because it means he doesn’t need a reality check to be sure she loves him the way he is. Even more, it’s shown that this even satisfies him to the point that his usual, desperate “girl hunting” subsides: When he tries to flirt with Labrys in the end of his story in P4U, he can’t, because it doesn’t feel right and sincere, so he just stutters. The only other time something similar happens in the series is when Ryôji hangs out with the FeMC; someone he has a confirmed, powerful romantic interest in. Teddie doesn’t need to flirt with her, because she never gives him a reason to feel like he *needs* to prove to himself that her affection is real.

The other way around, Teddie is very healthy for Labrys as well. Not only does he never make her feel uncomfortable about her body (because he gets what it’s like to have a constructed body) and is able to talk to her in depth and sincerely about what it means to crave the approval of humans as one of their own, because he’s been there as well (and often still is), he’s also very good at reading Labrys’ emotions and reacting to them appropriately, his brand of humor actually makes her smile, and when he *does* playfully flirt with her, she actually sort of enjoys it (she actually gets jealous when he offers to be Aigis’ knight as well, because she prefers having that all to herself).

The two of them build up each other’s self esteem, making the other seem welcome, useful and valid, while also having a very good dynamic and chemistry with each other. When Labrys tries to indulge in a Manzai (Wise Guy+ Straight Man comedy routine), Labrys always responds to it exactly as he needs her too, and they cheer each other up that way. She’s actually able to not only indulge his need for affection, but also his brand of humor, and likewise, Teddie is able to appropriately respond to Labrys’ emotions without needing to think about it much and he always knows how to make her smile.

This is a dynamic they don’t have with any other of their friends.

This is why I think they go so well together.