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Jherii will appear during the beginning stages of Chapter 5; her side mission is titled "Look in the Mirror". She is powerful and fairly resistant to ranged weapons, but not ranged attacks from melee weapons. If you get in too close, she can take out large chunks of your health if she swings her barbell around though. The best method here is to take one of your most powerful melee weapons, rush her, do a few hits and then roll away. Rinse and repeat.