I feel that Aya is more likely to be a manifestation of unresolved feelings both Koto and Erika have. She is not 'real' in the sense that she will stay on and have a happily ever after with one or both of them.
Hard to specify what makes me so sure of the above, but 1) she only appears when they return to their hometown and 2) she is not the personality she used to be, but more a projection of what Koto and Erika think a 14-year-old girl should be. Very 'psychological ghost'-y.
I foresee that Aya's reappearance will force Koto and Erika to figure out themselves, then Aya will disappear back into the great unknown.
Of course, being the author, Kabocha-san can do whatever they want.