A "returning fighter" refers to someone who has previously gone away, usually to fight in a war or conflict, and has now come back home. Full definition
During our time with the game, we were able to play as Rosalina and Palutena, two of the new additions to the Smash roster, as well as returning fighter Samus. (snackbar-games.com)
The trailer also shows other returning fighters such as Nightmare, Kilik, Xianghua, who all made their debut in the original Soul Calibur. (wccftech.com)
Bandai Namco is aiming to change that, as the developer has announced that SoulCalibur VI will include returning fighters Ivy and Zasalamel to the game. (gamingtrend.com)