A young schoolgirl has helped her friend stab her own classmate for as many as 19 times, a development which has caused quite stir.

A “deluded” schoolgirl helped a friend stab their classmate 19 times so the fictional Slender Man character would not slaughter her family, a court heard.

Anissa Weier and Morgan Geyser were both 12 when they allegedly knifed Payton Leutner in a frenzied attack and left her for dead in woods to please the creepy figure.

Despite being severely injured, Payton, then 12, survived and crawled out of the woods in Wisconsin, US, where she was found by a cyclist with wounds to her chest, stomach and arms.

Weier, now 15, pleaded guilty to attempted second-degree homicide in August and is now on trial over whether she should be held criminally responsible or committed to a psychiatric hospital.

Her lawyers have claimed Weier was suffering from a mental illness at the time and “lost touch with reality”.

Defence attorney Joseph Smith Jr yesterday told Waukesha County Court: "Anissa's broken mind caused her to lose touch with reality.

"Anissa was under the command and control of a delusional disorder."

He played segments of Weier’s police interrogation in which she described a plot to kill Payton in order to become a proxy of Slender Man.

She described the internet character as tall and faceless with numerous tentacles.

"He could easily kill my whole family in three seconds," she told the detective.

Smith described Weier as a loner who struggled to fit in with her classmates and who found a friend in Geyser.

While Weier was dealing with her parents' divorce, teachers began noticing symptoms of depression, he said.

The schoolgirl developed a "delusional belief system" with Geyser and together they made a plan to kill Payton and become Slender Man's proxies, Smith said.

Although Weier did not physically stab Leutner, in her mind she knew it had to be done, Smith told jurors.

Waukesha County Assistant District Attorney Kevin Osborne told jurors that Weier went along with the attack to preserve her one and only friendship with Geyser.

He said Weier knew the stabbing was wrong because she twice declined to do it herself as originally planned.

Instead, Weier told Geyser to do it. "Go berserk," was the command, Mr Osborne said.

"They knew this was wrong. They understood what they were doing was wrong," Mr Osborne added.

Weier’s dad William said his daughter went through a difficult time after his divorce.

But he never saw anything to suggest she needed mental health care.

"In my opinion, she was a normal child," he said.

One of the girl’s former classmates, who was also interested in Slender Man, told the court she had discovered how to become the character’s servant - by killing a friend.

The girl, identified in court only by her initials, Kn, testified that Weier then told her: "Don't worry, it's not you."

Both Weier and Geyser were charged with being a party to attempted first-degree intentional homicide.

Weier struck a deal with prosecutors in August in which she pleaded guilty to being a party to attempted second-degree intentional homicide.

But she also pleaded not guilty due to mental illness of defect, setting up the trial on her mental status.

Judge Michael Bohren told jurors they must decide whether Weier had a mental illness at the time of the crime and whether she lacked the capacity to understand her wrongful conduct.

Psychologists testified at a previous court hearing that Weier suffered from persistent depression and a delusional disorder linked to schizotypy, a diminished ability to separate reality from fantasy.

At least 10 of the 12 jurors must agree on a verdict.

Geyser has pleaded not guilty to being a party to first-degree attempted homicide. Her trial is set to begin Oct. 9.