In the Character Tab, you can create a character of any class at any time. To use a created character, add it to one of your decks and select that deck at the start of a match.
To deploy a character, you need Deploy Points. You can earn Deploy Points over time, by gaining a certain amount of Valor Points, or when your team's core is under attack. The higher the rank of a character, the more Deploy Points are required to deploy them.
During a match, you can press 'Warp' to deploy a new character or to warp to a tower controlled by your team at any time.