Board Thread:Questions and Answers/@comment-199.126.253.7-20131227130622/@comment-3250527-20140928153410

Yui is an AI, created and designed by Kayaba. Any similarities between the two are completely coincedential (sorry to trample on your hopes).

But again, in the The Day After side story, Sachi comes back as a ghost. The rest, I leave to you to hunt for.