once you get your fighter to the maximum level (30 for bronze), you'll get the option to evolve them. in order to do this, you need to sacrifice three of that same fighter (if you're evolving rerun, you can sacrifice rerun OR sketchy. just has to be a bronze peacock). like powering up!
after that, they'll be a level 1 silver fighter. you can evolve them into gold as well! it's just more difficult.