Allison is a girl who appears in Insidious Chapter 2 ending scene. Not much is known about her other than she is inhabited by a spirit.
Insidious: Chapter 2
After the Lambert case, Specs and Tucker, together with the spirit of Elise arrive to her house. Her family, consisting of her parents and her younger sister, inform the team of Allison's inability to speak as she is in an unexplained coma. As Elise entered the house secretly, her sister was the only one to notice her (suggesting a potential ability to astral project). Elise then approached Allison only to be confronted by an unseen figure behind her. A cracking sound is heard (implying the appearance of the Lipstick-Faced Demon). Elise sees this figure and gasps "Oh my God". Nothing else is known up to this point as this ends the scene in a cliffhanger.