Although she was never seen referred to as Aerith in game prior to
Kingdom Hearts, in the original
Final Fantasy VII, before naming the character she is called "Aerith" by default. This is never seen by the player since the first time her name is seen is after the name select screen, where the name selection's default is Aeris. If the name selection screen never appeared, then her name would be Aerith. The same applies for Cloud, who is called Ex-SOLDIER in the game, but this is changed when the player gets to the name selection screen where the default on the name selection is "Cloud". The same also applies for
Red XIII, who is just called "Red" before naming him, but unlike the latter two, this is actually seen at the top of a dialogue box during the player's first interactions with him, and changes to "Red XIII" in the name selection screen.