Everything that turns a character into more of an object rather than a person. This is more about the way they get treated, rather than how they feel themselves. So if you have some kind of doll suit and even though the character is still completely the same personality wise, they get treated as a doll / just an object.
Yukari's Portal Play by Ririka
Yukari’s Portal Play by Ririka (Archive Link) An absolutely amazing latex fetish story, involving quite a bit of portals. It plays with different Thouhou characters and their abilities to create portals or gaps. Yukari uses this ability to dress herself and her friend Yuyuko into some quite restricting latex gear, as well as making both of them basically limbless, by hiding their arms and legs inside such gaps. With multiple layers of latex gear encasing both of them, Yukari finally finishes their setup for quite the special outfit, making it look like nothing unusual is actually going on… ...