Tear Jerker / Dr. STONE reboot: Byakuya

• Chapter 2: There is a long argument between the astronauts on where they should land their pod. After much effort, Byakuya manages to convince everyone to land in Japan. When everybody is asleep, Lillian asks him why he so adamant on it. He admits to her that he just wants to see his son again.
• Chapter 3: Lillian does one last dance and song in space, because this is the last time humanity is going to go there. Byakuya is moved to tears.
• Chapter 4: Byakuya steps in to return to Earth, and Rei asks if they will return. Byakuya, wanting the robot to pass on peacefully, gives him a promise that he will return for it. You can tell from his expression that he knows it's an Empty Promise, and that it breaks his heart to leave his creation, something that he built with Senku, behind, but he has no other choice.
• Chapters 5 and 6 make this even worse. Rei interprets Byakuya's Empty Promise as The Promise and becomes a Determinator with the single-minded goal of keeping the ISS in space for as long as it can, until humans come back for it. The Dramatic Irony of knowing that Byakuya can't return to space, doesn't plan on doing so, and will inevitably die alone on Earth, makes Rei's efforts all the more sadder.
• Chapter 7 drives that point home and makes Byakuya's aforementioned death even more heart-breaking. Byakuya thinks that both the ISS and Rei are destroyed. He doesn't know that Rei is still alive and working on the ISS to keep it afloat in space. Rei thinks that Byakuya must have forgotten him and sends him a signal. As Byakuya lays down dying and thinks of his son, he sees the signal and realises that his Robot Buddy is still alive in space and waiting for him. Which makes his tears ever more tragic.
  • Rei then decides to keep sending the signal once every year, whenever the ISS passes over the island Byakuya has landed on, so that Byakuya knows that he is still alive, and knows where he is in order to get him. Cue the signal light shining over Byakuya's grave.
  • It's much Harsher in Hindsight, because it's established in chapter 3 that Rei doesn't know a thing about human lifespans (he told Darya Nikitina that she looks a thousand years old). So Rei is stuck working on the ISS in space, still expecting Byakuya to get him, not knowing his creator has long since passed away.
  • It's also quite Harsher in Hindsight for Byakuya too. Earlier in the same chapter some kids ask him about the ISS and Rei, and he tells them the story but mentions that both the ISS and Rei have been destroyed. The kids then present him with a flag for their newly-formed village and ask him what he thinks of it. Byakuya only addition is a crude picture of the ISS with Rei still inside. Byakuya never forgot about the little robot he and Senku built.
• The final few pages of chapter 8. Rei has sacrificed its resources and a big part of the ISS in order to save humanity from a life-erasing meteor strike. Due to the lack of resources it can no longer repair and upgrade itself and it's shown to be in a really beat-up condition. It's clear that both Rei and the ISS are on their last legs. And, yet, Rei is determined to keep the light signal going in order to guide Byakuya in space once more to fullfil his Empty Promise, despite the huge energy cost it has.
• Rei's final moments in chapter 9. Rei, about to cease functioning laments its existence, how finite the human is, and if it was all worth it. It then recalls its final memory of Byakuya and Senku back in chapter 1 and it concludes, that, yes, there is worth in a lifetime of waiting just before it shuts itself down for good.
  • Sure, Rei is later reborn as a Robot Girl, but we don't know if it maintains the same memories of Byakuya, Senku and the rest of the ISS crew, or if its personality is still intact. For all intents and purposes the Rei we used to know for the past eight chapters, the one who single-handedly saved all life on earth is gone.