Manga with main character waking up in a different world to see a girl pursued by a butcher wielding a gun with a parasite in it

Question

I remember reading this manga a while ago and when I went back to read it again, I couldn't remember what it was called.

I do remember in the first or second chapter that when he came to that world, he woke up on top of a tall building. When he woke up, he ended up seeing another girl jumping to her death to get away from a man in a butcher outfit wearing a mask on another building. The butcher soon started to chase after the main character, so the boy starts to run across bridges to get to other buildings where he soon finds the gun that ends up having what seemed to be the parasite in it. With the gun, I think he ends up killing the butcher and later finds the girl and dog. I also remember in one of the last chapters, he ends up fighting a person in a bunny mask and dress with the girl and dog on his side.

The main character had black hair and seemed to be quite young. The girl also had what looked like long black hair and looked to be even younger than the boy. The dog also looked like a small Shiba Inu breed. The parasite that he ends up getting looked like a black blob with an eye.

Answer 1

After looking into the answer Umineko Naku gave, I found that it has a sequel Tenkuu Shinpan Arrive (High-rise Invasion arrive).

The first chapter's title page has the main guy holding a gun, and has a girl with a small Shiba Inu. The first chapter ends with a girl jumping to her death, and we see she was chased by a guy in a mask in the second chapter, who then starts chasing the main character. The third chapter ends with the main character finding the gun that has the eye blob that then goes into the main character.

Answer 2

Perhaps you may be looking for Tenkuu Shinpan (High Rise Invasion)

Upon witnessing a man's head cracked open with an axe, 16-year-old Yuri Honjou trembles in fear and confusion as she flees from the masked assailant, only to find out she's trapped in an abandoned building where every door is mysteriously locked. Desperately searching for a way out, Yuri runs to the rooftop, but a world with no signs of life stands before her, surrounded by high-rise buildings. Though filled with despair, once she learns that her brother is also in this strange place, Yuri is determined to find him and escape. However, she soon finds that there are more masked murderers in the area, anxious to terrorize their newfound victims and satiate their sickest desires, leaving Yuri to question if they will be able to make it out alive.

Tenkuu Shinpan has many deaths, tall building and mask people. However, the main character for the first arc is female and I don't see much parasites, guns or dogs in the first few chapters after skimming through them.