The now 16-year-old, known only as Girl X, was visibly upset as she said that she saw teenage Boy Y knifing Brianna, also 16.
Prosecutors allege Brianna was stabbed 28 times in the head, neck, chest and back with a hunting knife after she was lured to Culcheth Linear Park near Warrington, Cheshire, on February 11.
Boy Y and Girl X, then 15, deny murder and blame each other. They cannot be named due to their age.
Manchester Crown Court jurors have seen a handwritten alleged “murder plan” from Girl X’s bedroom plus texts between her and Boy Y, purportedly on the idea of luring Brianna to the park and killing her.
Girl X wiped away a tear and said that any talk of harming the transgender teenager was “a fantasy”.
She said she had expected herself, Boy Y and Brianna to “hang out” in the park that Saturday and they stopped near a bench.
She said she walked off to stretch her legs.
Richard Pratt KC, defending, asked: “Did something attract your attention?” Girl X replied: “Yeah. I heard a noise like someone screaming and that’s when I turned around.” He said: “What did you see when you turned around?” She replied: “I saw Boy Y stabbing Brianna” and indicated on a photo of the scene that this was on a path near the bench.
Mr Pratt asked: “How many times did you see Boy Y stabbing Brianna?” Girl X said: “Around five times. I saw her fall and land face down on the floor. I saw him stood next to her, leaning over.”
Her lawyer said: “How did this make you feel?” Girl X said: “I was in shock.” He asked: “Did you want it to happen?”. The girl replied “No”. She had earlier admitted watching torture and killing material online, being interested in serial killers and having “dark fantasies” of killing other children, including Brianna – but said that she had no intention of making any of it reality.
Mr Pratt then asked her: “What did you do?”
Girl X replied: “I knew if I tried to step in, Boy Y could easily have overpowered me in that situation.”
She denied taking part or telling Boy Y to stab Brianna: “I walked over to see whether I could see her breathing. I couldn’t.”
She said Boy Y ran off – she “panicked” and ran with him: “I was scared if I tried to ask him he could come after me thinking I could snitch.” The court heard Girl X later claimed Brianna had been with her but left to meet someone.
Asked why she did not tell the truth, she said: “I was scared because obviously I knew how bad it would look…I was defending myself as well as Boy Y.” She said she sent messages to Brianna after she died as she was“paranoid”.
Mr Pratt asked how she felt about Brianna’s death, and she said: “I do feel really upset about it.”
The trial continues.
