After losing to the mid-Boss and starting a new game, speak to Plenair and go through the Dark Assembly Tutorial. Speak to her again. When your options appear, there will be a blank space above the "Exit" option. Press A on it and Plenair will say something similar to "They've found me..." and automatically join your party at level 1. She has incredible speed, uses guns as her main weapon, and already has the Nether 35 and an Imperial Seal equipped. Her first special attack is Usagi Drop, which is just about the same as Prinny Barrage but with bunnies.