Fun concept! Though I'm mildly disappointed the balloon wasn't dyed red with blood given the cover. I think the page where the guy started floating away could've been laid out a bit better to make it clearer he was floating away from the ground. I wonder why he and his girlfriend freaked out about the balloon first, did they see someone else try to leave and get transformed? And if so, why did they try to leave anyway? Did his girlfriend transform into a balloon without leaving?
Also, the ghost girl's frown was pretty creepy for such a simple frown. Tip for artists: if you want to make any expression creepier, draw all the individual teeth. The ghost girl also had individual teeth on the page when she asked the main character to find the balloon.
Story aside, I feel like the translation might be a little weak? Like on page 12, "A girl in a red hat. They are asking us to find a red balloon." I think it should say "she is asking" instead of "they." I don't speak Japanese, but I know it doesn't use pronouns and some lines can be interpreted/translated pretty differently as a result. So I'm also wondering if the final lines on Page 31 about "Ah... a balloon. We're rising. I wonder how far we will go." were meant to be third-person, like some random person seeing the balloons fly away. Those bubbles just felt so calm compared to the previous panels.
That said, thank you so much for translating and sharing this!!