Your sixteenth birthday here on earth is supposed to be something special.
For most people it is, for most people it is one of the most exciting days of their lives. Rob isn't most people though.
On your sixteenth birthday you get your Verba Amatoribus which is basically a tattoo of the first words your soul mate will ever say to you. On Rob's sixteenth birthday he didn't get his Verba Amatoribus. Rob is a solus.
Solus are not the most respected group of people on earth. Who are we kidding, mostly everyone on earth has a strong distaste towards the solus.
Rob went through the first twenty one years of his life alone, believing that he would be like this forever. That is until he met Preston, some random guy he met on the street, claims that they are soul mates.