Sword Art Online (ソードアート・オンライン Sōdo Āto Onrain?) is a Japanese light novel series written by Reki Kawahara and illustrated by abec. The series takes place in the near-future and focuses on various virtual reality MMORPG worlds.
Actually, that's an actual theory for Cloud's story--that only Sora/Donald/Goofy and Cloud can see Sephiroth. (Nomura was asked if this was true but didn't respond.) However, if this was true, then Tifa wouldn't really "exist" either. That would make sense since none of the other characters in Hollow Bastion talk to her--UNLESS, sometime during that whole Tron's world incident, if you talk to Leon he mentions her (just a line of text, no dialogue--easily missable). So..... maybe?