1. It's from Animal Crossing, and it's a marriage proposal.

2. Really don't sign it! It was probably sent by Light Yagami (though there are a few other options, but he's the most likely candidate who's at all close to the law), but whoever did, whoever's name is written in it will die! Like, 30 seconds later from a heart attack if no other details are given, if I remember right! It would be bad if you died.