I should have known this all along, but I was holding out hope that it would actually be a new character. That "mysterious character" in Dragon Ball Xenoverse is a custom creation. So, in essence, it's "you."
You'll be able to create your own character from parts using Earthling, Majin, and Namekian races (and more). Then said character will aid Trunks in an adventure that begins in the hub area known as Toki Toki City. And it'll look and feel ridiculous, because Trunks wouldn't just "recruit" someone.
With news of the "mysterious character" comes a closed beta announcement that we should be hearing a little more about in the coming weeks. I'm still disappointed, and I don't even know why.