The drunk teenager was among a gang of five who “savagely and mercilessly” attacked Sophie Lancaster, 20, and her boyfriend, Preston Crown Court heard.
Miss Lancaster was begging the gang to stop beating Robert Maltby, 21, when they turned on her in Stubbylee Park in Bacup, Lancashire, the jury was told.
The 15-year-old, who cannot be named because of his age, denies murder.
At an earlier hearing a 16-year-old boy, who was aged 15 at the time of the attack, admitted Miss Lancaster’s murder and admitted attacking Mr Maltby.
The accused, and four other youths, two aged 17 and one 16, have already pleaded guilty to grievous bodily harm to Mr Maltby.
The court heard Miss Lancaster’s facial injuries were so severe, paramedics did not know what sex she was.
Tests indicated she had been kicked and stamped to death, with the pattern of some footwear still on her head.
Miss Lancaster, a gap-year student, died two weeks after the attack.
Prosecuting, Michael Shorrock QC told the court the young couple, from Bacup, were walking home from a friend’s house when they “fell into conversation” with a group of teenagers.
The talk was friendly and good-natured, the court heard, but five of a group of youths “turned” on Mr Maltby.
The accused had started the violence, with a flying kick to the head of Mr Maltby, the jury was told.
The gang, “encouraging each other and laughing” punched, stamped and jumped on his head until he was unconscious, Mr Shorrock said.
As Miss Lancaster kneeled down, cradling her boyfriend’s head on her lap and calling for help, the accused and the then 15-year-old youth who has already pleaded guilty to murder, turned on her.
The second boy kicked her in the head, with the accused joining in, the court heard.
Paramedics found the couple lying side by side, covered in blood and unconscious.
Mr Shorrock said the gang had turned on the young couple simply because they were Goths or “moshers” and dressed differently to them.
Mr Maltby survived the attack in August last year but has not made a full recovery. He has no memory of the incident.
Mr Shorrock said the five, none of whom can be named, were “acting like a pack of wild animals”.
“The attack was totally unprovoked.
“It would appear that Mr Maltby and Miss Lancaster were singled out, not for anything they had said or done, but because they dressed differently to the defendant and his friend,” he said.
The case continues.