The unnamed young man only known as Rina's Brother, codenamed F-501, was part of the "Immigration" initiative from the 23rd century. He and his sister wound up inadvertently in an accident in the 20th century upon their arrival, causing chaotic events to unfold.
He was a young man with wild, light colored hair, the fringe of which was long and sweeping outwards.
When incognito, he wore a long coat, scarf, and dark pants. The coat and scarf were partially to hide the fact that he had highly advanced cybernetic arms, one of which he lost in a traffic accident.
From what could be seen of this character, he was mostly quiet due to the shock of the accident and their possible discovery, but seemed concerned about getting his lost arm back.
As later revealed, finding his arm was a life-or-death matter, which lead his younger sister to attempt to retrieve his arm however possible.
He was left helpless for the remainder of his appearance, nervous but unable to say or do much at all but scream when he was killed.
F-501 and his sister F-502 (Rina) had arrived at the 20th century in 1968 as part of the Immigration plan, but soon got into a car accident when a drunk driver swerved at them. Though they were not seriously injured, F-501 had lost an arm, which was later retrieved by the 00 cyborgs and taken back to Dr. Gilmore.
Although Rina attempted to retrieve the arm, she was unsuccessful, and arrived back at her and her brother's hiding location to find that the patrollers of the Immigration plan had arrived before her, ripping off her brother's other arm and threatening her for nearly blowing their cover.
The punishment for the "treachery" of F-501's cybernetic arm being left behind was thus death, and the F.P. agents killed him with their specialized guns, reducing him to little more than a scorch mark on the bedsheets.
Rina would later reveal that due to the aftereffects of WWIII, plants had evolved to be carnivorous and her brother lost his arms to a plant eating them.
- In the original serialization, as well as in the 1968 anime series, this character was depicted with fish scales having grown on his chest and he was born without arms. This was stated to be due to the nuclear fallout of WWIII causing many people to be born with deformities. The references to birth defects were deemed controversial and removed upon later reprints, with the scales on his chest being erased and his arms said to have been eaten by the plant. However, the F.P.'s bizarre, nuclear war-caused mutations remained.
- Anime adaptations have altered his relation to Rina, to have him be her boyfriend rather than lover.