Casca Highbottom and Crassus Snow
Dean Highbottom and his friend at the time, Crassus Snow (who was Coriolanus' father) invented the game, but did it as a joke whilst drunk. Unfortunately, it caught on, as is the way with Dr. Gaul:
From The Ballad of Songbirds and Snakes:
“Not in the districts. In the Hunger Games,” Snow corrected him. “I have you to thank for that. After all, you’re responsible for them.”
“Oh, I think half the credit for that goes to your father,” said the dean.
Snow frowned. “How do you mean? I thought the Hunger Games were your idea. Something you came up with at the University?”
“For Dr. Gaul’s class. Which I was failing, since my loathing of her made participation impossible. We paired off for the final project, so I was with my best friend — Crassus, of course. The assignment was to create a punishment for one’s enemies so extreme that they would never be allowed to forget how they had wronged you. It was like a puzzle, which I excel at, and like all good creations, absurdly simple at its core. The Hunger Games. The evilest impulse, cleverly packaged into a sporting event. An entertainment. I was drunk and your father got me drunker still, playing on my vanity as I fleshed the thing out, assuring me it was just a private joke. The next morning, I awoke, horrified by what I’d made, meaning to rip it to shreds, but it was too late. Without my permission, your father had given it to Dr. Gaul. He wanted the grade, you see. I never forgave him.”
Officialised by Dr. Gaul
Dr. Gaul was a vile woman who was obsessed with the idea of control and the humanity's tendency towards evil.
She took the idea that Dean Highbottom came up with and made it official:
“He’s dead,” said Snow.
“But she isn’t,” Dean Highbottom shot back. “It was never meant to be anything more than theoretical. And who but the vilest monster would stage it? After the war, she pulled my proposal out, and me with it, introducing me to Panem as the architect of the Hunger Games. That night, I tried morphling for the first time. I thought the thing would die out, it was so ghastly. It didn’t. Dr. Gaul took it and ran, and she has dragged me along with it for the last ten years.”