A girl accused of repeatedly stabbing her classmate to protect her family from an imaginary monster called Slender Man had a “broken mind”, her lawyers have said.

Anissa Weier, 15, and her friend and co-defendant Morgan Geyser, also 15, allegedly stabbed their classmate Payton Leutner 19 times in a park in Waukesha, Wisconsin, in 2014.

All three girls were aged just 12 at the time.

"Anissa's broken mind caused her to lose touch with reality," defence lawyer Joseph Smith told jurors.

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

Ms Leutner survived the stabbing and was able to crawl away for help. She was found by a passing cyclist.

An artist's depiction of Slender Man, a fictional monster which began as an online meme (LuxAmber/Creative Commons)

Ms Weier and Ms Geyser were arrested later that day and said they were walking to meet Slender Man in a forest in the north of the state.

The court was played sections of a police interrogation of Ms Weier conducted shortly after her arrest.

In it, she reportedly described a plot to kill Ms Leutner in order to become a proxy of the imaginary monster Slender Man.

She described Slender Man as tall and faceless with numerous tentacles capable of killing her family in a matter of seconds.

Slender Man originated as an internet meme and is believed to have been influenced by supernatural fiction.

It is believed the defendants, who are being tried as adults, read about the Slender Man character online.

Mr Smith described Ms Weier as a loner who struggled to fit in with her peers and who found a friend in Ms Geyser. While she was dealing with her parents' divorce, teachers began noticing symptoms of depression, he said.

With Ms Geyser, Ms Weier developed a "delusional belief system" and together they made a plan to kill Ms Leutner and become Slender Man's proxies, Mr Smith said. Although Ms Weier did not physically stab Ms Leutner, in her mind she knew it had to be done, Mr Smith told jurors.

Waukesha County assistant district attorney Kevin Osborne told jurors that Ms Weier may have believed Slender Man was real, but she had the mental capacity to know she was committing a crime. Mr Osborne said the initial plan was for Ms Weier to stab Ms Leutner, but Ms Weier couldn't do it and instead directed Ms Geyser to do the stabbing.

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

Mr Osborne said the police interviews showed it wasn't until after the attack had taken place that Ms Geyser told Ms Weier that her family could have been in danger.

"She goes along because she wants to preserve the one and only friendship" with Ms Geyser, he said.

William Weier, the first defence witness, testified that his daughter went through trying times in grade school as her parents divorced but he never saw anything to suggest she needed mental health care.

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

A former classmate of Ms Weier's who also was interested in Slender Man testified that Ms Weier told her one day that she had discovered how to become a Slender Man servant: By killing a friend.

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

The lead investigator in the case, Waukesha police detective Thomas Casey, also took the stand on Tuesday.

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Both Ms Weier and Ms Geyser were charged with being a party to attempted first-degree intentional homicide. Ms Weier struck a deal with prosecutors in August in which she pleaded guilty to being a party to attempted second-degree intentional homicide, essentially acknowledging she committed all the elements of the offense. 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 she had a mental illness at the time of the crime and if so, whether she lacked the capacity to understand her wrongful conduct.

Psychologists testified at a previous court hearing that she 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.

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