There are some spoilers ahead!
Now that that is out of the way, let’s talk about how you can unlock Yuuki in Sword Art Online Re; Hollow Fragment. First, clear out the Hollow Fragment portion of the game before finishing up Aincrad.
Then, go to Floor 100. Once you beat all the enemies in Floor 100, you will be given a prompt to return home or stay in Sword Art Online. We don’t want to return home so pick ‘No’ and you’ll be presented with some cutscenes.
Once the cutscenes end, you’ll be thrust into a battle with Yuuki, who is a pretty tough contender at low levels. She can use sword skills that can take away half of your health in one go, and if you’re too low then be prepared to meet a swift end.
Her starting level, with no 10 level increases from new game+, is 150.
She can parry your skills, not to mention if she’s running towards you that is a sure sign she will use a sword skill. Don’t be afraid to dodge although don’t remain close even if she’s using a sword skill – some of her attacks can hit behind her. Hyper armor and protective armor can save time, allowing you to make riskier moves without waiting for an opening.
If you’re high level like I was(203), then simply use Hyper armor and your ultimate sword skill when she’s using her own skill. My Nebula Empress completely destroyed her.
If you win, congratulations! Yuuki will be available to partner up with in new game+ and multiplayer.
If you don’t win, once the game ends and you’re taken back to the main menu, simply hit ‘Continue’ to fight Floor 100 again. You don’t want to hit last battle option as you won’t get the prompt for fighting Yuuki. In continue, you’ll be given the option to stay in SAO or return home once again. Alternatively, if you’re quick you can shut down the console before it saves and reload. I recommend the continue method as it reduces risk of save corruption.
Yuuki isn’t the only unlockable character, so you’ll want to stay tuned to see which one is next!