Here's a theory:
The reason for why Leaf never appears in the game if you choose Red or vice versa is because she IS Red, but in a AU. It isn't to farfetched either since Pokemon likes a lot to play with different realities and timelines (each version is a different universe, XY and RS being the biggest offenders), it would explain why when we re meet with Red and Blue in SM, she is nowhere to be seen, because she lives in a different universe and we and GF only follow the one where Red exists. Basically what I'm saying is, in a game where your other gender counterpart (ala Brendan/May or Lucas/Dawn) doesn't appear, they are the exact same person, but from two AU's
Tinfoil hat, off.
