Red = Generation I protagonist, also known as "Satoshi" (which would be localized as Ash)
Green (localized as Blue) = Generation I rival, also known as "Shigeru" (which would be localized as Gary)
Blue (localized as Green) = Generation I girl in a black dress commonly believed to be a scrapped female protagonist, would later be redesigned along with Red in FRLG and used as the female protagonist there. She has no standardized name as she never appears as an NPC, but many people settled on "Leaf" based on one of the potential default names for her in FireRed. However, that and many other names were just as applicable to her as they were to Red and Green based on internal data.
Blue (or localized as Green) should be her standardized name now.