Yes, it is a true story. There are real - life cases of sleepwalking like Mark Schall's, which often involve people performing various actions while asleep and being unaware of what they are doing.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
One possible reason could be that during sleepwalking, the brain's normal inhibitory mechanisms are disrupted. So, the sleepwalker may act out violent dreams or impulses without the normal self - control. Another reason might be related to underlying psychological stress or trauma that surfaces during the sleepwalking state.
In the sleepwalker killing true story, the killer was in a state of sleepwalking when the act occurred. Evidence had to be carefully examined to prove this state. The case also had a huge impact on the family and friends of both the victim and the perpetrator. There were many psychological evaluations done to understand if it was really a case of sleepwalking or if there were other factors at play.