F-501 started out as Rina's older brother in the original manga, another time traveler from the 23rd century. He lost his prosthetic arm at a crucial moment, which put both siblings at risk of exposure—so the Future Police tracked him down and killed him with a ray gun, wiping him from existence entirely.
The 1968 anime took a different approach, reimagining him as a character named Jack, Rina's boyfriend instead of her brother. He doesn't make it out any better in that version—he meets basically the same end.
The 2001 series brought back a similar design under the name Nichol (voiced by Tomoya Kawai), though this time without the lost limb subplot. He gets killed by his teammate Cain, who uses psychic powers to take him down.
Content compiled by AnimeList.moe from publicly available sources.
