Denise was living in the Alexandria Safe-Zone and remained largely to herself, presumably because of her anxiety. She had initially offered her support to Pete however he turned her down.
JSS
Denise is first seen studying in the clinic when Tara and Eugene come in looking for Aspirin. Denise insists on examining Tara when she complains about a headache. She reveals to them in casual conversation that she went to med school to be a surgeon, but had panic attacks due to the pressure, causing her to switch into psychiatry.
After the Wolves attack, Denise panics as Rosita rushes an injured Holly into the clinic. Tara and Eugene try to encourage her to help Holly as she bleeds out on the table. Denise ultimately agrees to help, but fails anyways. The guilt causes her to lash out at Tara. It is inferred that she stabs Holly in the head to prevent reanimation afterwards.