I'm going to say as my official answer "Cloud, Hojo, and Zack" all involved in the Shinra organizations Sephiroth clone experiment.
Hojo conducted the experiment of course.
Cloud vanished when he was 16? to go join soldier. Tifa knew something was up, but wouldn't tell Cloud directly.
Zack vanished 10 years ago from Gongaga, and Aeris knew but wouldn't tell the family.
Other than that, I'm really starting to dig hard and don't know if I could come up with anything else.
