Could've been structured better, but it was interesting.
I honestly never really understood the point of Chikara's research. He's building a robotic bird so he (and others) can fly. (There's an adorable and funny flashback with him being sick in bed for three days as a chid once he found out that people can't turn into birds.)
Once he becomes an adult, his professor and classmates scoff at him, because humans can't fly, duh! What an absurd concept. Which made me think at first maybe this was meant to be some alternate universe where airplanes haven't been invented yet...except not only are airplanes specifically mentioned later on, he's in an aeronautics program at university. So why is everyone so disdainful of his project?? Why is he reinventing airplanes?
I guess, with his backstory about his grandmother being trapped on their little island and unable to visit their grandfather in the mainland hospital before he died, it's meant to be a single person flight device, for someone to jet off in at a moment's notice. Which brings up some issues that I think should've been his professors' and classmates' main concerns...like adequate training for the pilots and air traffic control safety measures for all these people up in the sky...plus what's the fuel source?
But I don't even know where the humans were supposed to go in the bird he was designing. Inside it? On top of it? And since he was designing it at a drone-sized level, would the calculations really hold up to a larger version with a human passenger?
It was kind of messy science that I found pretty distracting. Plus I don't get how Satomi was doing medical research (?). How did his sensors cross from the medical field to controlling the wings of Chikara's bird? How were the two of them running into each other at school to begin with? I think maybe it was supposed to be a general engineering school with an aeronautics program, but even so, Satomi was specifically stated to be presenting at an aeronautics conference.
So overall, while the mechanical bird and Chikara's flight dreams were really cute, it didn't make a whole lot of sense and could've been adapted into something that'd fit the story better.
If you sweep that aside and focus on the characters themselves, the plot still struggles a little, but the dynamic holds up better.
While it's sort of built up as a slow-unveil spoiler, it's really important for the entire plotline to understand that Satomi's tattoos are meant to be shocking upon their first reveal, early into the story, when Chikara sees him through their adjacent windows.
While I know that tattoos are generally looked down on in Japan, due to their connections to the yakuza, etc, it still wasn't connecting for me until pretty late in the story that they were supposed to have basically ruined Satomi's entire life up until this point.
He was raised by an abusive, alcoholic father who sold him off to some debt-collectors, who seem to have just used little Satomi for fun tattooing sessions. (...Weird, and would anyone do this? Plus if you tattoo a child, how would that actually show up on adult skin? I don't know that much about tattoos, other than how they change on aging/sagging skin, etc, but wouldn't it stretch a lot with that much growth?)
Other weird part: it's kind of a half-finished tattoo of a phoenix, cut off at a clear line halfway across his back. At first, when I was misunderstanding what this story was about, and thinking it might've had some supernatural elements to it, I'd thought that maybe the rest of the tattoo would bloom as he was healed by his relationship with Chikara or something, and he might even sprout actual wings...
But no, it's just a normal human story about terrible human parents.
Chikara is the first person to ever be intrigued by Satomi's tattoo and to call it beautiful. Satomi has a ton of trauma around his tattoo - because as the author's note states, he would've been left out of a lot of activities during school, like swimming and class trips, and was always forced to hide himself from others. And adults who knew about his family history whispered about how it was in his blood to grow up to be like his horrible father - something that Satomi believed.
So when he met beautiful, bright, sunny Chikara, who had big dreams and a friendly attitude and who did brave things (like confronting a stalker on a train) while being terrified out of his mind, he got hooked. He loved being around Chikara. Chikara's presence and chatter calmed him down and drowned out that trauma-sound of the tattoo needles that's stuck with him since he was a kid.
Once you have all that backstory, the fast progress of their relationship makes more sense, as well as a decent amount of Satomi's actions. But I do think all of this wasn't conveyed as well as it could have been, so the story suffered some.
I liked the art, though, and their intimate scenes were done really well - uncensored, too. I liked how, by the final chapter, we were well past the "oh no was Satomi really a stalker all along" red herrings and got to see more of Satomi intentionally being so gentle and careful with Chikara because he was so afraid of being like his father. More of that type of content would've done a lot to bring this up to a four star story. It just got bogged down by trying to do too many things without really knowing how to accomplish them all.