Luke — Depression
This one should be relatively obvious. Luke's narrative throughout The Haunting of Hill House is one of the most devastating thanks to his emotional bond with his twin sister, Nell, and his gut-wrenching struggle with drug addiction. When Nell dies, his recovery is threatened in a major way and his life all but spirals out of control. Of all the Crain siblings, Luke takes Nell's death the hardest, and finds it the most difficult to cope. As he says during his speech at her funeral, he always felt like she was his "big sister" even though he was born first. He also tearfully tells Nell's ghost, "I don't know how to do this without you."