In order to get the full Jack of Blades outfit, you have to be evil through the whole game. In the very beginning, you have to be a bad kid. When you are given the choice to either let Twinblade live or die, kill him. Kill your sister and take the Sword of Aeons. Kill Thunder, when you need to collect the 'king of the arena soul'. Kill Briar Rose, when you need to collect the 'Heroine soul'. Finally, kill the Guildmaster, when you need to collect the 'oldest soul'.
At the end of the game, put on Jack's mask. If you do not get the whole outfit there is something wrong with your game.
There are many powerful people in this world. Few of them are responsible.