>>57285859To my understanding, they're different characters, both are based on the girl
>>57287129 posted. Leaf is the protagonist in a timeline where you choose the female character, that is her role. If you happen to choose Red, she's gone. The thing is that most official material (GSC, HGSS, USUM) states that Red is THE canon protagonist, the kid from Pallet Town who defeated Team Rocket and became champion. You can play as Leaf in FRLG and do all of that if you want to, she will be real in your universe, but Red is the one who is recognized as the official protagonist.
Green is a trainer from Kanto who didn't have the same journey as the FRLG protagonist. She did exist alongside Red and Blue in LGPE, but had a different role altogether.
Alternatively, masters Leaf says that she is Red and Green's childhood friend and the three of them got their first pokemon from Oak, and she did hang out with them such as that one time Red caught Snorlax. But that's a different universe from FRLG, because she doesn't actually show up as May/Brendan or Serena/Calem do in their respective games. Green's appearances were too brief to know for sure, her demeanor is different from Leaf anyway.