She is portrayed by Mara Wilson herself.
After the Critic mocks the film and Mara's roles as a child actress, she shows up and tortures the critic with his humiliating old high school videos. She procedes to mock him and embarass him while laughing evilly. Her eyes then glow demonicly as a captain reads "DON"T F%#K WITH MARA WILSON". She then returns in the Nostalgia Chick's review of Matilda, in which she tries to kill the Chick in order to get to the Critic, not realizing that he "died" in To Boldly Flee. After this, Nella takes note about her chest, and appears to reach out to grab it, only for the Chick to interrupt later.
As she has appeared in both intros for when Nostalgia Critic returned, it is possible that she may very well give him trouble in the future again.
- Near the end of The Critic's review of I'll Be Home for Christmas, A book called 'Where am i Now? The story of girlhood and actual fame' by Mara herself. possibly might mean that Mara might return to torment the Nostalgia critic again.