Saiteihen no Otoko (meaning 'Male of the Lowest Social Class') is the story of Murai Masahiko, a youngster who considers himself to be fortunate for one simple reason: he's not the absolute lowest social class male in his class. There is exactly one person below him in that regard, and there's such a wide gulf separating the two of them that he feels that things will never change... until that low class male gets a girlfriend, leaving Murai in a pinch. Murai shows the picture of a girl to his class, claiming this is his girlfriend. The next day, a girl, claiming to be that person appears in class. But, that's impossible... since that girl had been murdered several years ago. Now, in order to maintain the fiction that he has a girlfriend, and he's not the lowest class male, Murai must do whatever this mystery girl says... even as evidence is mounting that she is in no way human, and may in fact be a man eater in the darkest sense of the word...
- Abusive Parents: Murai's father. He's also the one who murdered the real Haruka years ago.
- Anyone Can Die:
- Murai is eaten at the end of chapter 4. Although, he wakes up alive at the start of chapter 5, with no idea what exactly happened.
- By the end of the manga, the only characters that are still alive are a stalker of Haruka named Yumi, and the police detective who had originally investigated the real Haruka's murder. Haruka, all of her "children", and Murai are killed when Murai eats Haruka.
- Cheshire Cat Grin: Haruka and others of her kind, whatever they are are prone to very unnerving smiles.
- Daydream Surprise: Murai's imagination gets away from him when the serial rapist and murderer he just tracked down and followed into a secluded alley suggests the former can help him satisfy the urges he's had to suppress while on the run. Turns out the guy doesn't want Murai to blow him; he just wants money for a prostitute.
- Depraved Bisexual: Given the fact that Murai has a very vivid fantasy about having to give a serial rapsit/murderer a blowjob...
- Dirty Coward: Murai. He basically becomes a "truffle pig" for Mizusawa Haruka, looking for low class males for her to eat. And if he does not find one every week... it will be his turn. Up until he decides that he can't go on like this, and decides to kill himself, but not before he takes out Haruka, as well as his father, who had murdered at least one person, and possibly others as well. It fails, but it's still the bravest thing he's done since the start of the series.
- Fat Bastard: Murai's father is a carbon copy of Murai appearance-wise, and is an abusive creep.
- Humanoid Abomination: Mizusawa Haruka, and possibly others.
- Idol Singer: The end of the manga revolves around an idol group created by Maya, one of Haruka's first "children" and a sleazy talent scout. Seeking out girls who are oddballs and losers, they are "changed" by the older members once they're brought into the group.
- I Kiss Your Foot: Murai is forced to do this as a show of submission twice, first by the foul-smelling Yamada, then by the considerably less offensive (but much more threatening) Haruka.
- Kill and Replace: The monsters' main activity. They target people that are troublesome, dirty, or otherwise "losers", and eat them, which somehow creates an "improved" copy of them, cleaning themselves up and becoming more polite. Disturbingly, the targets' families prefer the monster impersonating them because of this.
- Perverted Sniffing: Murai keeps and regularly sniffs a pair of Haruka's panties. He has a bit of a fetish for people's natural odors in general. He learns that his classmate Satou shares this fascination, and they bond over it, just in time to give Murai second thoughts about feeding her to Haruka, but too late to save her.