Complete the Hollow Area first, then beat the final boss on Floor 100. After the post-battle dialogue, you'll be asked if you want to investigate a suspicious area. Hit yes, and you'll meet Yuuki. She's level 150.
You need to get her down to half health before she gets you down to half health, so as soon as you get control use PRT Armor for temporary invulerability, use all the health buffs you have and then burn her down as fast as possible. She can dodge/parry your sword skills and then counter with one of her own, and it's pretty easy to get taken out by a single skill if you get caught out.
If you fail, you can Alt-F4 and re-open for another shot. If you don't want to do that, fight the floor 100 boss again after sitting through the credits and you'll have another chance.
This unlocks her as a party member, but you'll only get her when you trigger NG+.