source: wikipedia
As a child, Jun exchanged mementos with a young Tatsuya Suou, who was his playmate at the Araya Shrine. Jun gave him an engraved zippo lighter, while Tatsuya presented Jun with his father's silver wristwatch. Jun has carrying the watch ever since.
Jun has extensive knowledge of horoscopes and a great interest in horticulture, specifically towards flowers. He is fluent in hanakotoba, or "flower language", which he uses whenever he hands someone a specific flower. The symbolism behind the gesture can be good or ominous, depending on which flower he uses. His mother, Junko Kurosu is a selfish actress who cared more about revitalizing her career than taking care of her family. (and later became one of the Masquerade Executives, Queen Aquarius) Jun's father Akinari Kashihara, died before the game begins and is widely believed to have been a deadbeat parent and a crackpot. Following his father's death, Jun adopted his mother's maiden name, Kurosu.
Jun ran away from his mother's house at the age of seventeen and began to manipulate rumours in order to become the Joker; once he did, he acted under direct guidance from Nyarlathotep. He was rescued from this state by Tatsuya Suou and the rest of the party, and from then worked with them to undo the damage he had done while he was the Joker.