Morgan Geyser, 20, has asked a Waukesha County judge to order her release, in the same way he did for her co-defendant Anissa Weier last year.
The women were convicted of attacking their sixth-grade classmate in what they told a court was an attempt to appease the horror character.
Weier spent nearly four years in a mental health facility before she was allowed to go and live with her father and wear a GPS monitor.
Prosecutors alleged that Geyser and Weier attacked Payton Leutner at a Waukesha park after a sleepover in May 2014, when all three were aged 12. Geyser then repeatedly stabbed the victim while Weier urged her on. The pair left the victim for dead, but she was able to crawl out of the park where she was found by a passing cyclist.
The victim survived, despite suffering 19 stab wounds in the attack.
Police found Weier and Geyser later the same day walking along Interstate 94.
The girls told investigators they were going to Slender Man’s mansion in the northern part of Wisconsin and claimed they attacked the victim as it would make them Slender Man’s servants and he would not kill their families.
Geyser pleaded guilty to attempted first-degree intentional homicide and was sent to the Winnebago Mental Health Institute after the judge determined she had mental illness.
Weier, 20, pleaded guilty to attempted second-degree intentional homicide and was also sent to the psychiatric facility after a jury found she was suffering from a mental illness.