Chisato's Ex-Wife

In the first OVA episode, Takatsukasa Chisato is mentioned to have an ex-wife. Chisato makes another reference to her in the first chapter of the second volume, stating that he has already been married once. She remains unnamed and unseen. All that is known about her is that she was once married to Chisato, gave birth to Riju sixteen years before the series start, and divorced Chisato five years before the series start.

It's unclear why he and his ex-wife divorced, and she was not mentioned that often in the series so it's assumed after they divorced she died afterwards from either illness or an accident.