"Speaking of Claudius, he has a cousin, the Head Gamemaker this year. He's a couple of years younger than he is but he's quite a genius when it comes to making mutts and arenas."
-Jackson talking about Iuvartus in "An Ice Cold Killer - The 66th Hunger Games"
Iuvartus Templesmith is the main antagonist and the Head Gamemaker in The 66th Hunger Games to The 69th Hunger Games. He is also described as a kid that he was very spoiled. His father is Didius Templesmith. It's also said that he was a genius when it came to making mutts and arenas, as he put in lots of work to make the Games very exiting.
Iuvartus Templesmith was either executed or imprisoned after the events of Mockingjay