Anime First: The anime and manga began releasing almost simultaneously, but the former is the original and the latter the adaptation (anime has a longer lead time than manga). The manga's quite different, and lacks Hajime and Tsugumi except in omakes.
Defictionalization: Someone has actually made a real "Hell Correspondence" website. For those who seek vengeance, the site can be found here.
By the way, it doesn't work. Your target will not actually go to hell, so you need not fear having a certain black mark put on your neck setting you on the path to hell.
Not that the main cast other than Ai (see below) doesn't invoke this. Masaya Matsukaze not only does Ren live-action, he does the voicing too. Hone-Onna has the same voice as Anko Mitarashi, while Wanyuudo sounds like Zangetsu.
Many seiyuus weren't that popular when they played in this series. They got success after that.
And Nana Mizuki, not typecast for once, as Tsugumi in all three seasons. No, she doesn't get to sing.