Shimauma

Tatsuo, Hide, Satoshi and Aya worked together to cheat people out of money. They tended to target marriage services, where they con money out of people looking for love. They were good at it, until they crossed the wrong guy. Then they saw the world below the surface of dupes to cheat, when the 'collection agency' came for them with extreme violence.

The person who takes Satoshi and Hide, and then Aya and Tatsuo, is a wild-eyed maniac who seems to get off on the torture. There may be hope for Tatsuo, though, because the man recognizes him. The torturer is "Akasada" a bullied kid that Tatsuo knew from school. Now Tatsuo and Aya have been pulled, against their will, into Aka's world.

(Source: MangaHelpers)