You can earn EXP for each character by using them in matches. As you accumulate EXP and meet certain conditions, you can obtain higher-ranked 'Titles.' Setting these Titles makes your characters more powerful. You can check the progress of EXP and the conditions in the 'Change Title' section at the bottom left of the Character Customization screen.
The higher the rank of the Title, the more powerful and customizable the character becomes. However, characters with Titles cannot be deployed at the start of a match. You need to accumulate Deploy Points during the match and use them to deploy these characters. Deploy Points can be earned over time, by gaining a certain amount of Valor Points, or when your team's core is under attack.
Additionally, the higher the Title rank, the longer the respawn cooldown.