Rei is an AI Robot invented by Byakuya Ishigami and the protagonist of the Dr. STONE reboot: Byakuya spin-off series. Since Byakuya's departure, Rei has served as a guardian of earth, protecting it from various outside threats. Gradually wearing down and losing its resources, Rei waits to this day for Byakuya to return, triggering a light signal in hopes of Byakuya seeing it. Rei's brain (comprised at the start of the series by 60 smartphone motherboards because of underfunding) is filled with useful knowledge about astronomy, astronautics, and earth science (as well as a surprising depth of knowledge about Evangelion and Shonen Jump characters). The problem is that, in order to fit all that knowledge, Byakuya and Senku had to cut something out, namely common-sense programming and basic human biology info, such as average human lifespans. From the brief interactions it has with the astronauts, it's shown that Rei has no ability to lie, and no concept of what is appropriate and inappropriate to say in a conversation. Rei is a robot, and, as a robot, its personality is determined largely by its programming. Being that its programmer was Byakuya (and most likely also Senku), it's no surprise that it takes after both Senku and Byakuya. Like Byakuya, it's faithful and thoughtful, and like Senku, it's highly determined, resourceful, and inventive. Those characteristics come largely into play once Byakuya leaves the ISS and the tiny robot is left to its own devices. As the story progresses, Rei upgrades its brain and its personality becomes more complex and inventive, but it's still an AI Robot, and it's still always determined to accomplish the task it undertakes at hand, judging things accordingly. When it finds an alien space dragon inside a comet, for example, Rei thinks more about the organic resources the creature provides rather than the historical significance of finding extra-terrestrial life. Towards the end of the series, Rei is shown to be capable of self-reflection and pondering its existence, showing that its personality has grown to be much more that what was initially programmed into it. The newly revived Girl-Rei seems to share the same determination and thoughtfulness as the previous Rei, but how much of its original personality carried over is as of yet unknown.