Ef: A Fairy Tale of the Two./WMG

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.
 * Poor Mizuki, hit by a bus between seasons...
 * Seemingly Jossed. Renji and Chihiro live in Otowa, Australia. Hirono and his group live in Otowa, Japan. Kei finally gave a visit to Chihiro in Melodies.
 * Not jossed. Now we know where Moe characters go when they die - Australia!
 * Whoa... Fremen-tan!

Yuu's relationship with Chihiro is an attempt at Wife Husbandry.

 * Yuu seems to only be able to be attracted to girls who "need" him.
 * Yuu mentions that Chihiro is similar to Yuuko.
 * Yuu is clearly trying to forget about Yuuko.
 * Therefore, it's likely that Yuu hoped he would be able to fall in love with (and probably eventually marry) Chihiro once she got older, allowing him to finally get over Yuuko.
 * Yuu's attempts to dissuade Renji are probably as much an expression of jealous possessiveness as they are an expression of lack of faith in their relationship's potential.
 * This also creates another parallel between Yuu and Amamiya's character's, which the writers are so fond of highlighting (note also that the warnings Yuu gave to dissuade Renji from pursuing Chihiro were similar to Amamiya's initial attempts to dissuade Yuu from pursuing Yuuko.)