Monster Musume (Monster Musume no Iru Nichijou, often translated as Daily Life with Monster Girls) is a romantic comedy/harem manga series by Okayado, and the anime started streaming in July of 2015. The core premise is to play with the Cute Monster Girl trope and some harem anime tropes for good measure.
The basic plot is that legendary monsters such as centaurs, harpies, mermaids, et cetera are all real and have only recently begun integrating into human society (think foreign exchange students, only more so). Enter Kimihito Kurusu, a part-time worker who ends up "hosting" a lamia (snake-person) named Miia more or less by accident. Later residents of his house include Papi the harpy, Centorea the centaur, Suu the slime, Mero the mermaid, Rachnera the spider-woman, and Lala the dullahan.
Oh, and every single girl there wants to jump Kimihito's bones.
Rather surprisingly for an ecchi series, there's actually a good deal of education to be found. Okayado researches zoology in order to write the various liminal characters, and they often have more in common with animals than just their appearance. Such things include the taste-sensitivity difference between carnivores and herbivores, the fact that spiders get drunk on coffee, racehorse breeding practices, the circumstances and consequences of snakes shedding, and many more subjects.
The main protagonist. A young man who's left to take care of the house while his parents are abroad, he finds himself saddled with (at the time of this edit) seven beautiful monster women who are all vying for his attention. Most of the conflict in the series is the fact that he can't choose, and the pressure placed on him to pick a girl is stressing him out considerably. The rest of the conflict is him getting hurt by the girls not knowing their own strength.
Personality-wise, he's a very kind and open-minded individual. His niceness and selflessness (often to the point of putting himself in danger) is the main reason most of the girls fall in love with him.
In badfic, he's often replaced by a character from another continuum. When he does appear, though, he's very difficult to get wrong due to the surprising simplicity of his character. The most common badfic error with Kimihito is getting his name backwards; Kimihito is his given name, Kurusu is his family name.
The first girl to move in with Kimihito, Miia is a lamia (snake-woman). She's clingy, sometimes jealous of the other girls, and very forward. She's also a terrible cook.
Typically, Miia is presented as normal in badfic, and she is rarely OOC.
The second girl to move in with Kimihito, Papi is a harpy. She is, quite literally, a birdbrain; she's fairly dumb, forgetful, and childish. Despite this, she has great maternal instincts, and unsurprisingly gets along well with children. As a harpy, she looks a lot younger than she is, for the sake of flight.
Much like Miia (and the rest of the girls), Papi is usually completely in-character in badfic.
The third girl to move in, Centorea (sometimes called Cerea) is a centaur. She likes to think of herself as a knight, with Kimihito as her noble master. Most of the humor with her involves her massive chest, which her clothing is usually fighting a losing battle with.
Suu is an illegal immigrant, as she wasn't brought in by the exchange program. She's ambiguously intelligent, devoted to Kimihito, and rather touchy-feely with other characters when they get wet. She's the basis for the minis of this continuum; see their page for more information on Suu.
The fourth girl to officially move in, Meroune (sometimes called Mero) is a mermaid. She's a fan of the Little Mermaid, and seeks a tragic romance for herself. For some reason, people tend to treat her like royalty once she starts talking.
The sixth girl to move in, Rachnera is an arachne (spider-woman). She's the first signal that the MonMusu continuum isn't all sunshine and roses, as she was essentially sold by her host family. She's bitter and resentful, although she opens up once she meets Kimihito. She also has a bondage kink.
Rachnera is the most popular character thus far.
The seventh, and so far final, girl to move in is Lala the dullahan. At first, she's presented as a grim reaper figure; it turns out that she's just really delusional.
For spoilerrific reasons, Lala is aware of the PPC and is willing to aid them... provided that her head and body are in the same place.
A coordinator for the cultural exchange program. She's irresponsible, lazy, and very frequently mooches off of Kimihito, and she's also the entire reason that Kimihito has so many liminals in his house. Despite this, she's actually fairly competent, and rather intimidating when ticked off. In Chapter 10, she reveals that her irresponsibility is due to having far too much work with not enough pay.
In badfic, Smith's irresponsibility is often exaggerated to absolute incompetence. In addition, due to scanlation errors in the first volume, she's frequently and erroneously called Sumike or Sumisu.
Monster Ops: Neutralization (a much better name than Okayado's original Monsters of a Neuro... thank you, translators) is a special operations team that works with Smith, and was put together to deal with liminal crime. It's comprised of Zombina the zombie (who's hot-headed and has more in common with Frankenstein's monster), Tionishia the oni (who's really, really girly), Manako the cyclops (the team's sniper, and also very shy), and Doppel the doppelganger (who's implied to have some connection to Nyarlathotep, the Crawling Chaos).
Aside from Doppel, the team has learned of the marvel that is Kimihito Kurusu's kindness, though they have not been added to the harem.
Starting in Chapter 17, Kimihito's household began to meet other liminals that do not get added to the harem. They've got lives of their own, after all.
The first is Polt the kobold (dog-person). She's a fitness freak who owns a stadium and a gym designed for liminals. In her first appearance, Kimihito and company are testing her facilities before they are officially open to the public. She tends to appear whenever sports, exercise, or competition show up in the story.
In Chapter 18, the "D" mystery begins, and the story temporarily adopts a Monster of the Week formula as one-off villains begin to make their appearance. There's Draco the dragonewt (a psycho lesbian with a crush on Miia), Kii the dryad (planted and forgotten by Papi, and hates humans for pollution), and Lilith the devil (who exists solely as an outside-context force of malice).
In Chapter 31, Yukio the yuki-onna is introduced. She fell in love with the son of her host family, and is working to help them with their onsen. Much like Polt's gym, Smith brings the Kurusu household here to test the facilities, as Yukio wishes to make the onsen accessible to liminals. She also comes with a few Frozen references.
In Chapter 32, Liz Ninetei the nine-tailed fox is introduced. Her host is a destitute Shinto shrine-keeper, and she desperately wants to help him. She works as a shrine maiden, but Miia and Rachnera help her attract visitors with a magical girl theater.
Aside from Kimihito and Smith, humans tend to not take center stage in this series. You'll definitely notice them when they make an effort, though.
The Racist Couple is introduced in Chapter 2, and they've been a pain in readers' behinds ever since. They're not just jerks to liminals, but to other humans as well. The man in the couple graduates to full-on evil when he attempts to rape Centorea in Chapter 16. Rachnera's retaliation currently graces TV Tropes' Nightmare Fuel page for the series.
The Director (who operates under the false name of Kasegi) is introduced in Chapter 14, who exploits the fact that Japan's censorship laws do not apply to liminals, and sexually harasses the harem while supposedly making a documentary on the Interspecies Cultural Exchange Program. When he wants to steal Papi's unfertilized egg, Kimihito puts him in his place (and destroys all of his footage) with one solid punch. His day doesn't get any better; he's the man that Rachnera's host family sold her to, and she ties him up so he can be imprisoned.
On the subject of Rachnera's original host family, Ren Kunanzuki, the daughter of said family, is introduced in Chapter 26 trying to get Rachnera back. While she claims it's out of remorse, she still sees Rachnera as a challenge to be surmounted, so when she asks Kimihito to transfer her back, he refuses.
In Chapter 25, we see a nicer side of human-liminal relationships, when Kimihito takes the girls out to the market to shop for dinner. The various stall vendors are thrilled to have the girls around, especially when Mero and Centorea appreciate their wares, as well as having Papi and Suu around to play with their kids.
Sadly, some fanwriters don't see the much deeper story to Monster Musume, and are distracted by the lucky bastard with seven girls that all want to sleep with him.
The general formula for MonMusu badfic is a crossover character switch. Typically, this means that Kimihito is replaced by a character from a different continuum. Because these generally follow the base plot of Monster Musume, however, the canon characters aren't usually that OOC, often acting exactly as they did in canon.
A common charge in MonMusu badfic is timeline disruption. Often, the girls are introduced in the wrong order, or events are altered in such a way that makes no sense with the canon.
Agents Native to This Continuum
- Kala Jeng - A girtablilu (scorpion-woman) in the DF who wandered into HQ by accident.
- Gabby - A hobgoblin recruited to the DMS, and later transferred to the ESAS.
- Publica Kurusu - A badfic recruit, supposedly the mother of a Kimihito replacement. Currently a Nursery Worker.
- Stephanie Podd - A cuddly kraken (squid-woman) recruited to the DMS.