While, to the casual observer, this guy may look very similar to the existing hero, Jim Raynor, he's a slightly different kettle of fish. If you've played the Wings of Liberty campaign, Tychus first appears in the very early missions, you see him being released from prison in his old armor, with his cigar miraculously remaining intact. It was part of a deal in which he would assassinate Kerrigan. Not to throw too many spoilers around for people who haven't played the campaign, but Tychus is likely not to have too many friends among the other Starcraft heroes.
Nonetheless, he's a prominent character in the SCII universe, and as he's renowned for saying, "Hell, it's about time..."