|“||Choose me and find happiness. If I can only have you, Kotarou I won't think about the rest of the world||”|
–The voice talking to Kotarou in Akane's Route
The Voice is a mysterious character only heard by Kotarou in Akane's Route.
After, in Akane Senri's route, she becomes the Holy Woman and the situation in Gaia worsens, Kotarou Tennouji hears a mysterious voice mocking him while he's pondering. He immediately identifies it as being exactly the same as Akane's voice.
Kotarou tries to go out to the roof of the Martel building to no avail, since it has no roof access, trying to get some fresh air. When he does, he hears a mysterious voice that sounds exactly like Akane's mocking him, talking about various things that only Akane and him would know. Particularly, it brings up how Akane wants to end her life and the world with it, and that things are hopeless.
Due to the fact that the voice mentions the Key being the true salvation of the world, it's highly improbable for this voice to be anyone but the collective being that is the consciousness of the Holy Woman. This theory is even more supported by the fact that Kotarou receives these words from above--meaning, from the place where Akane will attempt to recreate the Key's song of salvation later on. By this logic, "Choose me, not Akane." would mean choose salvation--choose the destruction of the Key's song, and not saving Akane.