Jurors will need to determine if the 15-year-old Weier was insane at the time she helped a friend stab a classmate in 2014, in allegiance to the fictional Slenderman character.
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 trial is set to start in October.
Weier and Geyser were both 12 years old when they lured their classmate, Leutner, into the woods in the Milwaukee suburb of Waukesha and allegedly stabbed her.
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.
Doctors said Payton Leutner nearly died in the attack.
Jurors are still trying to decide whether a Wisconsin girl accused of helping stab a classmate to please horror character Slender Man was mentally competent during the attack.
Weier had already pled guilty for her role in the stabbing.
Weier is accused of stabbing classmate 19 times with another girl
Because the jury found Weier mentally ill, she is considered not responsible for her actions in the attack. Leutner managed to crawl out of the park and was found by a passing bicyclist. Van Rybroek is the second court-appointed expert to evaluate Weier's insanity plea.
In his questioning of Trussoni, Assistant District Attorney Kevin Osborne focused on the fact Weier didn't clearly tell anyone she was afraid of Slender Man's wrath until after the stabbing, reinforcing his comments in opening statements Tuesday that Weier understood that what they were doing was wrong, but that she went along with the plot to preserve her friendship with Geyser.
Mr Van Rybroek said as a result Weier "couldn't stop herself at the moment it mattered most", and helped stab Miss Leutner even though she knew it was wrong.
Prosecutors say a Wisconsin girl accused of helping stab a classmate to appease online horror character Slender Man participated in a cold and calculated attack.
"First of all, she was 12", Westendorf said, adding that the teen was influenced by a website focused on imaginary killers.
"I know now it's just teenagers who really like scaring people and making them believe false things", Weier told Waukesha Detective Michelle Trussoni, according to the Sentinel. Geyser, who did the actual stabbing, is scheduled to go on trial next month.
Jurors saw clips of Weier's police interrogation, during which Weier appeared to move from belief in Slender Man to doubt to a realization that he wasn't real.
Geyser has pleaded not guilty by reason of mental disease to being a party to first-degree attempted intentional homicide.