Well, it could be that in the fanfiction, Cato has a secret admiration for Katniss all along. When they are in the arena, he finds a way to protect her from other threats without her knowing it at first. As the story progresses, Katniss discovers his actions and they form an unlikely alliance. They then use their combined skills to outwit the Capitol's cruel twists in the Games. This would show a different side of Cato and a new relationship between them.
Perhaps in this fanfiction, there is a twist where the Capitol decides to pair Katniss and Cato as a team from the start. At first, they are both resistant and distrustful of each other. But as they face the challenges in the arena together, they start to understand each other better. They learn about each other's families and the reasons why they fight. Eventually, they become so united that they are able to break free from the arena and start a revolution against the Capitol, inspiring other districts to follow.