The youngster, Girl X, was just 15 when Brianna, 16, was knifed 28 times in a park.
Now 16, she admitted having “a fantasy” about killing Brianna, which she had discussed with her co-accused, Boy Y, before the murder.
She told Manchester crown court: “I was making up a fantasy but I had no intent.”
The girl, of Warrington, Cheshire, who met Brianna at school, admitted she found her “attractive”.
She also confessed to having a browser on her phone which let her access “violent stuff” on the dark web.
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She also had an interest in serial killers.
Both Girl X and Boy Y deny murdering Brianna in Warrington on February 11. Trial continues.
Brianna Ghey’s body was tragically found at Linear Park in Culcheth on February 11.
She was from the Birchwood area of Warrington.
The 16-year-old’s family described her as ”a much loved daughter, granddaughter, and baby sister”.
In a statement, her loved ones added: ”She was a larger than life character who would leave a lasting impression on all that met her.
“Brianna was beautiful, witty and hilarious. Brianna was strong, fearless and one of a kind.”