- "Change, 00!"
- ―Rei's Transformation Call[src]
Rei/Kikaider 00 was the very first and failed Kikaider prototype . He was deemed a failure before his completion, and was left unfinished in Komyoji's lab, until he was completed by the priest who was taking care of Ichiro. Like Ichiro, Rei lacks a conscience circuit, but unlike Ichiro, Rei does not show any emotion.
Unlike his brothers, he neither drives a motorcycle nor has a musical instrument due to his late completion. He wears a yellow suit in his human form. He never appeared in the live-action series. Despite being the first to be built, he is considered a younger brother to Jiro because he was completed later (anime) while in the manga Jiro built him making him more of a son.
Due to being created in a rather simple way compared to Ichiro and Jiro, Rei shows little to no emotion towards things. Although Ichiro and Jiro did care for him as their younger brother, Rei did not seem to have any familiar bond with his siblings and acted rather coldly towards them. His compassion towards others in general was also lacking, which was shown when he stated towards Mieko that the emotions she felt were just programmings.
When Jiro and Ichiro were fighting more of SHADOW's members, they encounter another robot that looks similar to them, who to their surprise, was helping them. Explained by Futen that the robot in question was Rei, the prototype Kikaider that Komyoji left unfinished until Futen himself completed. As night time came, Jiro and Rei had a talk, with Rei telling Jiro to keep an eye on Rieko, showing his distrust towards her.
The next day, Rieko told them about her hometown, which led to the Kikaider brothers falling for a trap. They then learn from Gill that Rieko was actually a robot, which was confirmed when Rieko turned into her robotic form. Seeing Rieko suffer caused Bijinder to betray Shadow, with Jiro and the others escaping from the trap. Despite Bijinder's assistance, Akira got captured and Rieko got destroyed, causing them to grieve for her loss.
Talk with Mieko
When Mieko was sitting alone thinking about the emotions she felt, she realized Rei was spying on her. Noting that she isn't surprised he doesn't trust her, Mieko asks Rei why she feels these emotions, despite being a machine. Due to his nature, Rei coldly dismisses it as a programming, while showing little to no compassion towards her.
Battle and death
In the battle to stop the Armageddon weapon, Rei and Mieko were captured, leading to them having a Submission Chip installed. Going after them, Jiro tried to attack Gill, but was stopped by Ichiro and later by Rei, causing him to question their actions. Asking Gill what he has done to to his brothers, Gill taunted him and told Rei and Ichiro to stop but not destroy Jiro, despite Jiro yelling Gill's name in anger.
Asking his brothers to stop but to no avail, Jiro was defeated and captured. Learning from Gill that he added a Submission chip inside them as well as inside himself, Jiro cursed Gill in anger. While Gill installed a Submission chip inside Jiro to make him into a slave, the chip only conflicted with the GEMINI Circuit and Jiro ends up temporarily losing his morality. He then tricks Bijinder into releasing him as he destroyed his enslaved brothers without remorse then Gill as well. The Armageddon generator then explodes bringing an end to SHADOW's organization. With Akira being sent to Futen by Bijinder (who was seemingly influenced by Rieko's spirit) before she died, the day has been saved, but at a high cost
Abilities and Arsenal
Despite being the least developed out of the Kikaider brothers, Rei had the most firepower out of all his siblings, surpassing those like Ichiro, Jiro and Saburo, with even Jiro being surprised at his strength. His calm fighting style literally made him a fighting machine, one that had no mercy towards his opponents.
- Double Slash: Rei deploys blades from his hands and using his speed, slices his target apart.
- Blow Up: Rei's body transforms into a weapon. He deploys two Gatling guns from his chest, opens his legs to reveal missile launchers, his fingers turn into gun barrels and his fore head opens up to reveal three mini turret cannons. He then fires them all at once to destroy multiple targets.
- Unnamed Energy Blast: Rei could fire a destructive green energy beam from his hands.
- Though he never appeared in the Tokusatsu series, Rei may have been based off of another Shotaro Ishinomori character: Robot Detective K, as the Blow Up attack is similar to the Blow Up form.