Null Magical Girl Introduction: Year 2X21 C.E
First chapter, translation credited to sanshuowo:
Present day, present time, Jan/Mar 2021 in the middle of the pandemic. Basically it revolves around this 24 year old Kosane, out of college, unemployed, depressed. She had a twin sister, Eruna, who died at birth or a bit after. But she remains as Kosane's imaginary friend until Kosane hits 14 on 2011, when the Tohoku earthquake happened. Eruna disappeared, and since then Kosane's been living half-dead and almost-suicidal. Back in present time, Kosane's living like a slob with an attitude, the kind of person who will put coffee powder in her mouth and then pour boiling water directly in. She wakes up in the evening after a weird dream. Turns out she's smashed her head on the kitchen mirror and is bleeding. She goes to check in the bathroom, and surprise, she's cracked a hole in her skull and she can see inside. Apparently however, there's nothing inside her skull. She remembers that she contracted with Kyuubey 10 years ago. But the contract failed. Kyuubey shows up and tells her she's been the subject of their study, and now that she remembers, he comes out in the open. Apparently not only does she have no brain, she also has no soul. That's why her contract failed and thus Kyuubey calls her the Null Mahou Shoujo. Not only does she not have a brain nor a soul, she also has a wee tumour of Incubator cells growing from her tailbone. Kosane speculates she's an Incubator clone, which Kyuubey denies.
Also spacetime is warping around Kosane for some reason. Kyuubey requests cooperation to observe her but Kosane asks for an equal exchange for them to tell everything they know. From now on, Kyuubey lives with Kosane. Specifically, Kyuubey literally crawls into the hole of her skull and sits there. Apparently he likes it.
Some time passes and the spacetime distortion starts to become very apparent. Kyuubey is about to leave, but he finds that someone left a letter in Kosane's head. A physical letter. The letter is from Eruna to meet her. If Kosane folds the letter along the dotted lines into an origami tetrahedron, it becomes a ticket to their meeting place. Kosane sucks at origami, so Kyuubey, with literal paws, has to help out. The origami is folded and it looks there's an endless regression of tetrahedrons inside. And surprise, Kosane turns into a tetrahedron (Kyuubey is still comfortably in her skull).
Weird stuff happens and then she's on a train platform. She boards the Time Loop Express No. 9, and there are 5 mahou shoujos in there with shadows for faces. They approach her and she freaks out, and that's the end of Part 1. tl;dr she's the Null Magical Girl because she has no soul so the contract failed(?). We're not sure if her wish was granted.