Rento Makina is Kamen Rider Kikai, a Kamen Rider from the year 2121 who appears in Kamen Rider Zi-O.
Rento Makina was originally one of the Humanoise who conquered Earth, but chose to switch sides and defend humanity from the Humanoise after befriending a group of children.
During one of his precognition dreams he had while taking a test, Sougo Tokiwa was teleported to the year 2121 where he met Rento. After Rento's satellite was hacked to revert him to his original Humanoise programming, Sougo was forced to fight against him to protect the humans. However, Sougo was able to restore Rento to his true self before saying goodbye to Rento and the children and awaking in his present time and finishing his test.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga