Sister Yvonne, also known as Felicity, was a nun at St. Francis Orphanage and later a cultist. She was killed by snake bites.
When a lunar eclipse occurs the following night, a worried Yvonne confronts the Mother Superior about it. Mother dies of a heart attack when she beats Yvonne for this, and a distraught Yvonne, realizing that Delia is evil, turns from God and flees the orphanage.
Yvonne subsequently follows the Yorks and watches over Delia over the following years, but eventually stops when Delia fates her to die. At some point, Yvonne becomes a prostitute, but then eventually moves to North Carolina under the alias of Felicity, where she becomes the prophet for a religious cult.
One night eight - nine years after Delia's adoption, while Yvonne is stepping into a pit of snakes at a cult meeting as a test of faith, Earl Knight confronts her about Delia. The snakes then bite Yvonne numerous times, poisoning and hospitalizing her and leaving her in agony.
The next day, Earl visits Yvonne in hospital, where the latter tells Earl that Delia is connected with the Beast. Shortly after Earl leaves, sunlight from a window casts a shadow in the shape of an inverted cross over Yvonne, who then dies of her snake bites.
- When Delia bit into her doll's head upon seeing Yvonne, the bite marks this left on the doll's face appeared to mirror the marks Yvonne would develop on her face from fatal snake bites.
- Moments before Yvonne died, sunlight from the hospital window cast a shadow in the shape of an inverted cross over her.
Yvonne at first described the baby Delia as a "miracle," but after she discovered who Delia was, she came to fear even the memory of Delia.
The Mother Superior initially comforted Yvonne after they gave Delia to the Yorks, but when Yvonne became increasingly worried over who Delia was, she became more angry and rude towards Mother Superior. She was visibly horrified when Mother Superior died.