Diana Walter

Diana is a malevolent entity and the main antagonist of the 2016 horror film Lights Out. Originally a human in life, she became a vengeful spirit after an experiment went awry.

Early life
At age 12, Diana was taken to a mental hospital after she had manipulated her father into killing himself via mind games. She was diagnosed with a rare skin condition that caused her to react negatively to light, and she had violent tendencies. While at the mental hospital, Diana met and befriended Sophie, though as Becca pointed out, she most likely manipulated her mother into thinking that they were friends.

Eventually, experiments were conducted on Diana with healing her skin condition as the objective. Unfortunately, the experiment goes awry, and Diana is reduced to ashes. Since then, her spirit had latched itself unto Sophie, and only becomes manifested when she was off her medicine.

Present day
Some years later, Sophie's husband, Paul was conducting research into the experiments, as to flush Diana from her mind. However, Diana kills him, thus making Sophie believe that he had abandoned her. Throughout the film, Becca and Martin attempt to free their mother of the malevolent spirit's power, which invokes the wrath of the deceased girl. Diana began to manifest herself as a sentient shadow who could only be propelled by light. As such, the two siblings attempt to keep the lights burning throughout the house; however, a power outage results, causing the children to enter the basement in order to restart the generator. Diana then slams the door on the two, and she knocks Sophie unconscious when she was trying to take her medicine.

Brad, Becca's boyfriend, manages to get to his car, and he alerts the police about the seeming domestic disturbance. Unfortunately, the two police officers get effortlessly slaughtered by the vengeful ghost, and she attempts to brutally murder the two children, thus going against Sophie's demand of sparing them. Regaining consciousness, Sophie obtains a gun, and she reminds Diana that without her, she couldn't manifest in the physical world. Diana lunges at Sophie as she pulled the trigger unto herself, and she shatters into pieces. The film ends with the ambulance arriving to tend to the protagonists' wounds. Without any warning, the lights began to flicker, but Brad assures them that Diana is truly gone.

Trivia

 * Alicia Vela-Bailey portrays the ghostly Diana, whereas her human counterpart is portrayed by Ava Cantrell.