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.
The first time we see Rei, it's in the form of Version 23, a boxy, clunky prototype which is supposed to use wind propellers to move in zero-gravity conditions. As it's a prototype, it's rather messy in appearance and misses a lot of its later features. Energy is provided by a rechargable 12V Li-Ion battery, a couple of cranks serve as its arms, and a simple rectangular gyroscope is located inside to provide balance. Eyesight is provided by a couple of simple cameras on top of the robot.
The next version of Rei we see, Version 37, is a much sleeker, more rounded robot which, instead of propellers, uses air blasts to move around in zero gravity. It has a spherical round main body with a couple of gyroscope "legs," which help balance the robot and also house its hands. A couple of round censors are located on top, probably to serve for spatial recognition and sound input, while its eyes are comprised of two high quality circular cameras. That appearance remains pretty much the same throughout most of the mini-series, up until it finally ceases to function in chapter 9.
At the end of the final chapter, Rei is revived, or rather rebuilt, into the body of a beautiful robot girl, which is the same girl model the audience sees in chapter 1. As a robot girl, Rei has beautiful long hair that covers most of her body. The hair and the body contain various metal plates while her neck contains a couple of holes that probably function as input sockets. The circular sensors remain on top of the head and show up as hair decorations. Like the initial robot girl model, it shares the inscription "Rei 01" under its neck.
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.
Rei's first memory is that of Byakuya showing it off to Senku. Senku initially thought Byakuya was working on an actual robot girl (one with blonde hair, a pretty face, and the inscription "Rei 01"), but Byakuya quickly tells him that this is just a fake plastic model and not an actual robot, to which Senku comments how sad and lonely Byakuya must be to build that on his spare time. Byakuya then shows Senku the actual Rei, then-Version 23, a clunky, boxy mess of a prototype, and begins to apologize on how "boring" the actual robot is. Rei v23, however, gets Senku excited (much more excited than Rei 01, in fact), and it's implied that the two of them worked on the robot together.
Rei is present during the petrification event and makes its debut to the astronaut crew soon after, revealing itself as version 37, a much more rounder and slicker version, that Lillian Weinberg immediately thinks is very cute. Shamil is less than impressed, however. It is later revealed that while Rei has the brain-power of 60 smartphone boards and is filled with useful scientific knowledge, it lacks common-sense protocols and human biology information, such as average human life spans (telling Darya Nikitina that she looks over 1000 years old, much to her chagrin).
Rei is pivotal in returning the astronauts back to earth, providing the necessary scientific data and doing all the necessary calculations and simulations it order for the Souyz Module to land safely in an appropriate position. When it completes its calculations, it records Lillian's final dance and song in space as well as Byakuya's promise to her that this is not the end and that humanity will one day return to space.
The initial calculations were somewhat off and the initial crew inside the Soyuz Module (comprised of Lillian, Shamil, and Connie) goes several kilometers off-course. Refusing to let his comrades die, Byakuya prepares for another emergency landing near the place the Soyuz Module landed in order to rescue his friends in time, to which Rei also provides the calculations. Just before Byakuya, Yakov, and Darya get ready to leave, Rei asks its creator if it's also a friend and if he'll ever return to space to get it. Byakuya, with sorrowful expression, tells Rei that it is his friend and he will one day return for it.
Byakuya's promise was actually a lie. The astronauts needed all of the station's fuel to go back down on earth, and the International Space Station would, on its own, burn into the earth's atmosphere in about a year from the current date. Byakuya knew all that and never intended to come back for Rei. The promise was intended for the little robot to pass on in peace.
Lacking common-sense protocols, Rei instead takes Byakuya's promise literally and takes it upon itself to maintain and keep the ISS in orbit until Byakuya's return. To that end, it modifies itself to first fix the solar panels by attaching itself to the space station's arms. When it finds out that the astronauts used all of the station's fuel, it finds a way to modify the food they left behind into fossil fuel to return the ISS back into its original orbit. And when that runs out, it modifies the station's engine to be hydrogen powered.
As the new hydrogen engines needs water and Rei can't get water from Earth, Rei looks up into space for fuel instead. It tracks a comet and makes a "spaceship" comprised of the left-over space suits in order to come near it and retrieve it. It manages to successfully retrieve a big piece of the comet and successfully comes back in time to take care of the ISS. The comet, along with water, also provides numerous other resources that Rei finds useful in maintaining the ISS, such as minerals and ammonia.
At some point, Rei discovers that there's something wrong with its brain. Of the 60 smartphone boards inside it, 12 have ceased functioning. This was because the high energy particles coming out of the sun can burn delicate circuitry. Not to be one to give up, it uses the station's computer circuits to upgrade its brain with the capabilities of 1'000 SP (smartphone points) to match up its upgraded functionality.
20 years after the petrification event, Rei wonders how Byakuya is doing. Unbeknownst to it, Byakuya still talks about Rei and the ISS, but thinks that both Rei and the ISS burned up into the earth's atmosphere. He then draws a picture of the ISS with Rei inside it for the flag of the newly formed human village.
Back in space, Rei faces several problems, as several parts of the Space Station have ceased functioning and there is a lack of replacements. Ever resourceful, Rei comes up with another solution: collect the parts of the satellites that were left over by humanity in space. Rei goes on another resource hunt, and, when it finishes, upgrades its brain to that of "over 9'000 SP" (clearly referencing Dragonball Z and the infamous meme).
42 years after the petrification event, Rei, realizing that Byakuya must not know it's still OK and in space, decides to make a light signal to show Byakuya where it is in order to him to return. Unfortunately, it's too late. The first light signal Rei broadcasts is the last thing Byakuya ever sees. Realizing his little robot friend is still waiting for him aboard the ISS in space, Byakuya starts crying as he lies down dying alone in the river, unable to tell anybody that his little robot friend is still alive. Meanwhile, Rei, not knowing that Byakuya has just died, and having no knowledge of human lifespans, decides to show the light signal whenever it passes over the place Byakuya landed, but only once a year because of the high energy cost it has. The next time the light signal is broadcasted, it's shown lighting Byakuya's flower-covered gravestone.
200 years after the petrification event, Rei notes how much greener the earth looks. It then witnesses Mount Fuji erupting, turning a huge part of the formerly green land gray (meaning a large part of the life died). At the same time, it witnesses an asteroid hit earth. The asteroid lands in Rome and turns the city into a 25km crater. It also causes severe damage to the ISS and Rei is shown having a really tough time with the repairs. The asteroid impact has a severe ecological impact too, turning the entirety of Europe gray and ushering in a new ice age. Rei hopes Byakuya is okay and that he keeps himself warm as he lights up the signal once more.
It's been 470 years after the petrification event and the piece of the comet that Rei originally retrieved has evaporated, due to the sun. Rei goes on another comet-hunt and retrieves a much bigger extra-solar comet that it stores in the sun-earth's langrangian point (i.e. the point where the earth's shadow is constant) to prevent evaporation. The comet also houses an unusual find, a huge extra-terrestrial behemoth beast. Not grasping the significance of such a discovery, Rei decides to use the creature as "organic resources." It then sets up an asteroid detection system so that the ISS can move in time, should another meteor approach the earth.
Time flows for earth and the ISS. The robot witnesses numerous Mount Fuji eruptions and wonders if it can stop them. It witnesses Byakuya's island growing larger due to an earthquake and it replaces the solar panels of the ISS for the 50th time, for which it prints a commemorative book, like Shonen Jump's 50th anniversary. Rei wonders when will Byakuya return as it continues fixing the space station.
3200 years after the petrification, Rei discovers that a meteor is heading towards earth, set to strike Japan in 400 years. The meteor is 40km in diameter and weighs 300 trillion tons. Its destructive power is calculated to be 200,000 times more than the last one. Should it hit earth, not only will Japan get wiped out, but all of Earth's oceans will evaporate and all life on earth bigger than 1cm will go extinct. The entirety of earth will become gray.
Rei, now branding itself an Ishigami, decides to go crafting in order to prevent the meteor impact in order to protect Byakuya and the rest of humanity so that they may return. It creates a space factory that manufactures 4,000 hydrogen bombs which, 350 years later, get simultaneously launched towards the meteor. As the missiles hit the meteor, the explosions become visible on Earth, and Senku, in his petrified state, is shown watching the spectacle. It's unclear though if he understood what was going on.
The operation is successful and the meteor impact is averted, but in doing so, Rei sacrificed a big part of the ISS, and a huge chunk of its gathered resources. It can no longer fix and upgrade itself and it's shown to be in a really bad and beat-up shape. Rei still saved the light signal though and it's still determined to keep lighting it, despite the energy cost it has. As the light signal broadcasts once more, we are shown that the human survivors have setup a "star sign" acknowledging the light signal, but being unable to interpret what it ultimately means.
3,572 years after the petrification, Rei gathers up whatever scarce resources it finds around earth's orbit and constructs a giant robot, which, in actuality, is a 3D printer. Knowing that its end is near and that it will soon cease to function, Rei constructs a set of instructions, made out of physical discs, set to last millions of years, which will guide the 3D printer into rebuilding both the ISS and Rei directly in space, so that Byakuya can return to them after the original Rei and the ISS are gone.
As the memory of Lillian's final song and dance begins to fade, due to memory damage by solar radiation, Rei sets the 3D printer into motion. As it lays down, about to cease function, it wonders how many years humans live, and if the fact that the earth is now green holds any significance. It wonders if Byakuya will return for it tomorrow. It decides it's best not to know these things because waiting is for it a reason to stay alive. It then recalls its very first memory, that of Senku commenting on how sad and lonely Byakuya must be to make a robot girl model. Rei wonders if Byakuya is lonely and if that's a bad thing. Its final thought is that there is still meaning in a lifetime of waiting.
3,718 years after the petrification event, on the very day Senku is revived, the reconstruction of the ISS is complete and Rei is reactivated inside a new body. The new body resembles that of the robot girl "Rei 01" model Byakuya had in his office, and is much more humanoid in appearance. The newly rebuilt ISS now resembles a space city more than a space station. Still being beholden to Byakuya's promise, Rei gets back to sending the light signal.
Back on Earth, Senku sees the light coming from the ISS. The thought passes through his mind that it might be the International Space Station, but he immediately dismisses it and gets back to making the antibiotic for Ruri. As far as he knows, both the ISS and Rei went down ages ago.
Chrome, on the other hand, pays close attention to the star, and notices a pattern in the light signal. He notices that the light shines in different places each time. He then arranges the light signals the way he sees them and plots out the points. The points, when put together, read "Byakuya, I am here," but sadly, Chrome, not knowing how to read Japanese, doesn't know what they mean. Chrome then gets back to helping Senku and the sign lays down on his hut's floor.
The story concludes as the light signal goes off once more. Rei is still waiting for Byakuya inside the space station.
- Version 23: Under the development stage. Designed to propel in zero-gravity.
- Version 37: Rei's present form aboard the ISS when the petrification occurred. A supercomputer with processing power comprised of 60 smartphone chips
- Version 38: 380 days after petrification event, Rei upgrades itself to become gas propelled.
- Version 39: 2 years after petrification event, Rei upgrades itself again.
- Version 40: 8 years after petrification event, Rei upgrades itself again, buy upgrading its brain from 60 smartphones to 1000 smartphones.
- Version 41: 38 years after the petrification event, Rei upgrades itself again by upgrading its brain to 3,000 smartphones.
- Version 47: 
- Version 53: 
- Version 54: 
- Version 55: 
- Version 87: 
- Version 92: 
- Laser Gun:
- Reigelion Spaceship:
- Byakuya Ishigami: Being his creation, Byakuya acts towards Rei like a doting parent, something that Shamil states openly. From his expression as he gives his final promise to Rei, it's clear that it breaks his heart to leave the lttile robot behind, but he has no choice. Rei, on its part, shows some empathy towards Byakuya, choosing to remodel itself as a robot girl, because Senku commented on how lonely Byakuya was. It should be noted that at some point, Rei calls itself "an Ishigami," meaning it considers itself part of his family.
- Senku Ishigami: The relationship between the two is unclear as the two are not shown to directly interact with each other. Since Rei was a project that Byakuya completed in his University, Senku most likely played a part in upgrading its design it to the much sleeker version 37, and perhaps also programmed its operating system/personality. Or perhaps the robot just plainly recorded Senku's interactions with Byakuya and stored them as part of its operating protocols. What is clear, however, is that Senku had a strong influence on Rei, since the robot references him, his ingenuity, and his determination numerous times throughout the mini series.
- Rei 01, the robot girl plastic model we see at the start, is a reference to Battle Angel Alita.
- Rei's brain being made out of smartphone boards is most likely inspired by the US Air Force once making a supercomputer from numerous PS3. It should be noted that there are projects which are designed to turn a cluster of smartphones into a supercomputer, but they are still in their larval stage.
- Its name and self-made spacesuit references its naming from Rei Ayanami from Neon Genesis Evangelion.
- Appearance-wise, versions 37 and onwards of Rei share a lot of common features with Boichi's robot avatar. Like Boichi's avatar, Rei can change facial expressions, change head decorations, and display symbols and messages on its main body according to the occasion.
|Chapters||Reboot: Chapter 1 • Reboot: Chapter 2 • Reboot: Chapter 3 • Reboot: Chapter 4 • Reboot: Chapter 5 • Reboot: Chapter 6 • Reboot: Chapter 7 • Reboot: Chapter 8 • Reboot: Chapter 9|
|ISS Crew||Rei • Byakuya Ishigami • Lillian Weinberg • Shamil Volkov • Connie Lee • Darya Nikitina • Yakov Nikitin|