Although only mentioned a few times in the manga, Rio Hikari is an important part of the background story. Rio, the mother of Satoshi Hiwatari, was shown only in an undetailed way in the manga. She was also mentioned by Daiki Niwa when he was explaining to Daisuke Niwa that when he was a child, the Hikari was a woman who gave birth to Rio, then vanished. Rio then had a son, Satoshi. Is told by Kosuke Niwa in the third volume of the manga that Rio Hikari has been dead for about fifteen years, and it is hinted later that either Satoshi or Krad were to blame for her death.