According to an interview in the upcoming Dengeki Playstation, a weekly Japanese magazine, Final Fantasy XIII character Sazh Katzroy will most likely not make an appearance in the sequel, but may show up in planned DLC down the line. When asked whether Sazh would be in FFXIII-2, a Square-Enix official said; "Final Fantasy XIII-2 will continue after release, so look forward to later." Though cryptic and confirming absolutely nothing, the statement implies that Sazh, and any other characters missing from the main game, may still return.
Side-Note: Dengeki Playstation also reviewed FFXIII-2. They gave the game a "S" rating, their highest mark.