Jimin was a bright child. His family was ok. One day, his school went to a picnic at the Flowering Arboretum. His parents were the only that didn't come. He felt weird, but ended up still having fun. At the end, everyone was gone, it was just Jimin and a teacher. The teacher told him to wait for his parents, and left Jimin alone. Jimin thought he could go home by himself. He got to the back of the arboretum, and saw an open storage room.
We don't know exactly what happened here, but it is suspected he witnessed murder. It is also thought it might be a child who was in Seokjin's class, who was always being taken by mysterious men. This traumatic experience caused him to experience Dissociative Seizures. These would occur when his brain shut down from an emotionally triggering situation, like PTSD. He also seems to have depression and anxiety. Because of this, he was in and out of hospitals constantly.