Katniss was reaped for the 75th Hunger Games, the Quarter Quell, wherein a relative of the selected tributes would also be put into the games. Katniss and Prim went along with Gale and one of his younger brothers. 

Thankfully, all four made it out alive and started the rebellion with the help of Johanna Mason, Finnick Odair, Haymitch Abernathy, Plutarch Heavensbee and even Cato Hadley. 

You know the rest, the war happened, Finnick and Prim passed, and Alma Coin and President Snow were both killed. 

In terms of Panem, it recovered. New laws were made, citizens became better fed, led healthier lives, and the economy changed drastically in favour of the districts under the careful watch of President Paylor of District 8.

In light of new laws and the lift of the inter-district travel ban, Cato moved to District 12 so he could be in the home of his lost love, and with her family too. Katniss and Gale were married, and Cato became the godfather of their child. 

Cato never quite moved on from the promise of "what if" once given to him by a girl he met under tragic circumstances. He was never married, never had kids of his own, but he had family and he kept them close. He was as happy as he could be after everything life threw at him, and he wasn't afraid of death, because he knew it would mean a certain someone would say hello again. In her memory, Cato co-founded the Y/n L/n Foundation alongside Katniss Everdeen, an organization dedicated to feeding those who remained in poverty. 

As for me? I linger where dead people do, and I wait for the day where I can be with my family again, but I'm not by myself. After all, I haven't been alone all this time. I've had Rue, Prim, Peeta, and even my father to keep me content and joyful. But as for my story and how it finally ends, how I met Cato again in a better, safer place? I guess everybody knows.