Renji, Chihiro and the rest of their little clique are all dead.This explains a lot. It explains why they (Renji, Chihiro, Renji's mother, the playboy and the priest) all seem to know one another, but never interact with the other half of the cast. It explains why Hiro and Chihiro, formerly inseperable, now never speak of one another. It explains why they only hang out in ruined train stations, windswept beaches, deserted streets and other places where there are no other humans. It explains why Kei and Chihiro live apart. It explains how several days can pass for Renji in the space of one Christmas night for Hiro. It does not explain why Kei is still receiving texts from Chihiro on her cellphone, but if you've expanded your definition of normality to include ghosts, it doesn't seem unreasonable that these ghosts also have a phone plan. They died in the earthquake that pulverised the town, and they don't (or Chihiro doesn't) know it.
Yuu's relationship with Chihiro is an attempt at Wife Husbandry.