Haymitch calls Katniss "sweetheart" from the very beginning, but Sunrise on the Reaping explains she's not the first he's bestowed it upon.

There have always been a lot of questions about the 73 years of Hunger Games before Katniss and Peeta were selected as District 12’s tributes. Some answers came in the books that followed The Hunger Games,

Haymitch Abernathy’s time in the Hunger Games is at the center of Suzanne Collins ’ newest prequel book, Sunrise on the Reaping. Set 24 years before the events of the first novel featuring Katniss Everdeen (played by Jennifer Lawrence in the film franchise) and Peeta Mellark ( Josh Hutcherson ),