Each and every character found in Fortnite has the ability to evolve, and a number of times no less. However, the rarity of the character will actually dictate how many times that can evolve: Uncommon Rarity (3 Evolutions), Rare Rarity (4 Evolutions), Epic Rarity (5 Evolutions), Legendary Rarity (5 Evolutions), Mythic Rarity (5 Evolutions). Like most of games, evolving a character will see their stats increase significantly, giving them access to new abilities as well.
Each and every character will have a different set of requirements in order to evolve, so you will need to go to the Armory in order to take a look at all of your characters. To see a characters specific Evolution Requirements, first click on the character and then select Upgrade/Inspect. From here, click on View Evolution and you will be able to view the requirements for that specific character for yourself.
Each time you look to evolve a character, you will need to meet the Max Level as well as have a certain amount of Hero XP, Pure Drops of Rain, and Training Manuals. You will even have to have the required nodes purchased from the Skill Tree before you can evolve. In order to see what they hard, go into the Skills Tab and open Skill Tree: Tier 1. Click to the right of the Skill Tree until in branches into different sections, taking notice of the Number 2 inside of a star at the end of the branches. These are Evolution Nodes, and you must first unlock the paths to them by purchasing the nodes that lead to them.