So Atlus posted this on their Japanese Twitter account. It says "Spoilers are absolutely off limits. Especially everything from December onward, got it? For those of you foolish enough to spoil it, a fate worse than death awaits". While this just refers to not posting videos of new stuff on NicoNico and Youtube. They also did this with Catherine but didn't really do anything to people who uploaded stuff.
That being said; here's the spoiler discussion thread for P4G
According to 2ch, the new character Mary is a real 失格人間