Date was a doctor who worked at the same hospital as Shiki Murasaki.

Spoiler

He was involved in a medical malpractice in the hospital and after realizing Murasaki was willing to assist with the cover up out of her respect for him, he manipulated the hospital management into making her a scapegoat in order to protect himself. Murasaki entered his name into the Btooom! reality game as a result (although whether he knew this or not remains unclear)