After the "Mother Claudia" main quest you unlock Kirito mode. To access it you need to go to the title screen and select "start game" and select you save file. You can then choose to enter "Kirito Mode" which is a temporary mode only available between the last two main quest. Your locked in as Kirito, but the rare item drop rates are drastically increased and all the maps are accessible besides the last main mission areas. This make it easier to get some of the best guns in the game as rare drops form bosses.