Text Messages from Unknown

Once a upon a time there was a six friends who were best friends. But one day one of the six friends passes away from an unknown illiness. The five friends who are remain alive were very sad that their friend has passed away from an unknown illiness. But what the five friends didn't know that their dead friend was alive but in the shadows. The friend who is supposedly dead starts leaving text messages but leaves their first letter of their name.
Text message reads:
Why are you sad? Aren't you happy that I'm gone? Friends are we not? We will meet soon - S