Each character has a number of different titles to earn throughout the game. Some can only be received at specific sections and some will take multiple plays of the game. If you do not get them all on your first play, be sure to spend 500 in the Grade Shop to have the titles carry over to your next playthrough. Make sure you read about them before starting your game as some have multiple parts at various points and can be missed. Also be sure to do all the Secret Missions in a single playthrough to get one Yuri's titles.
Check this thread for a walkthrough of every title for each character.