It's a harem manga with a human boy and monster girls. Kimihito is going to be living with a lamia, a harpy, a centaur, a slime girl, a mermaid, an arachne and a dullahan. (Most of those are from Greek myth, but the dullahan is a Celtic headless horseman, there are mermaids in lots of cultures and the slime girl is original.) There will probably be more. That's just the harem so far. What's more, almost all of the girls have the aggressive hots for our human hero and I think every single chapter is full of mega-nudity and lurid sexual situations. Some of the latter are basically porn.
Yup, it's one of those manga. You'll need a rhino's hide (or to be porn-deprived) to get through any chapter without abandoning it as an affront against humanity. However if you can get past the glaring obstacle of what it is, I think it's quite good. I enjoyed it.
Fundamentally, it's a story about acceptance. Monsters have been accepted into our human world, but in a weird, grudging way that leaves room for prejudice and exploitation. It's called an exchange program. Monsters must have a host family and will be arrested if found anywhere on their own, even if it's not their fault, e.g. their hosts had sold them to a third party. Such a host family won't be deemed to have broken any laws, of course, and would even still be eligible to be hosts again in future. We also see traffickers who'll kidnap monster girls, which in Chapter 20 happen to be naked children.
It's not all quite that unequal, mind you. There's a "no sex with humans" law that will see the human sent to prison and the monster deported, but that on its own will melt your brain if you start thinking about it. (Of course these laws are really just writer's fiat for the sake of harem nonsense, but the story is explicitly examining the consequences of these laws and starts pointing out the loopholes very early.) Had any of the girls ever raped Kimihito, he'd have been the one sent to jail. Conversely, Centaura in one chapter is nearly gang-raped and is legally barred from fighting back to defend herself. Had the attempt succeeded, Centaura would then have been deported for breaking the "sex with humans" law, i.e. being punished for being raped.
Oh, and some of the monster races (lamias, harpies) are single-gender. They need humans to reproduce. (You can roll your eyes, yes.) The implication, though, is that mankind has just made it illegal for these species to breed. Well, short of artificial insemination, anyway.
Our human world can be racist towards monster girls. Cute monsters that look human-ish will be fine. However that doesn't apply to most of our heroines, e.g. Meer has an eight metre long snake body, Rachnera is a spider the size of a horse, etc. There's quite a lot of material addressing the girls' insecurities and issues as a result of this, with Rachnera being particularly damaged (and also scary). What makes Kimihito special is that he genuinely doesn't give a toss about species and just sees them all as girls.
He calls them a family. He even finds them sexy, including their monstrous halves. He likes legs, for instance. This is still true even if you have more than the usual number, they're covered in chitin and they end in spikes that can punch holes in cars.
While I'm discussing Kimihito, I should also mention that he has to earn his harem hero status in an uncommon way for this genre. Any of the girls could kill him in a heartbeat and he not infrequently gets sent to hospital just because for being in the same room as them. Meer can and does constrict him in her sleep, which breaks ribs even if he does get away alive. Sue can drown you just by getting too close and pulling you into her slime body. Standing near Centaura is as safe as you'd expect of a hot-tempered horse. Don't get in the pool with a mermaid. And those are just the girls who don't mean anyone any harm and would be horrified if they actually hurt anyone, which excludes Rachnera.
Another thing I really enjoyed is the wholeheartedness of the monster angle. The manga-ka's not trying to be shy about it. These are monsters and that's the selling point. Thus the girls will have an attractive and mostly human-looking upper half (almost always buxom), but the lower half will be all kinds of whacked-out freakiness. Meer's snake body. Wow. Papi the harpy looks dinky and harmless from most angles, but those eagle feet are like a velociraptor's. As for the mermaid, she's more authentically piscine than you'd expect, with pelvic fins as well as the main caudal tail.
There's also real-world biology. Instead of just handwaving its monsters as magical, the manga is trying to think through how creatures of these physiologies would interact with the world. Spiders really can get drunk on caffeine, for instance. Carnivores, omnivores and herbivores do indeed have different senses of taste, which is why Meer's cooking is so horrible. Cold-blooded species can easily get into life-threatening situations. Manako the cyclops has no depth perception.
The most important thing with any harem show, though, is of course the girls. I don't mean that in a salacious way. Does their characterisation work? Do you care about them? Are they cliched? Does the story manage to sell their attachment to the harem hero, or does it come across as contrived and/or forced?
Here, I thought the manga made it work. The fact that it's basically porn is a problem for anyone who's trying to read it for its non-pornographic qualities, but apart from some compressed time spans (i.e. Meer falling in love with Kimihito in three days) the characters come alive and their relationships are believable. It also helps that only two of the harem are properly in love with him, with the others being less serious about it and/or dealing with other issues.
The girls are:
1. MEER (lamia) - one of the two Serious Girls. She was also the first, which generally gives an unspoken status in this genre. She's by far the most emotionally intense, gets jealous (unlike the others) and gets a lot of character exploration. Personally, I'd assume she was The One if it weren't for the fact that she's the most dangerous to be near. As far as her lower body's concerned, all her friends are just sources of heat and/or prey animals.
2. PAPI (harpy) - theoretically she's one of the core three girls, with Meer and Centaura, but in practice she's a loli-appeal birdbrain whose feelings towards Kimihito tend to look little-sister-ish. She looks underage, but that's because birds have to be small and slight for flight and in reality she's the same age as everyone else. (A stroke of luck for the paeophile audience!) She also has a memory that resets itself whenever she takes three steps, giving her the attention span of a three-year-old.
"Oh, that's right. Papi can't see anything in the dark. I forgot." -- says Papi, flying in the dark.
Her physiology also often causes inconvenience, since she has no hands. She has wings, not arms. Being able to fly makes up for that, though.
3. CENTAURA (centaur) - the other Serious Girl. She thinks and talks like a medieval knight at armour and has no sense of humour at all. She's bound up her honour entirely with Kimihito (long story) and will be ferocious in his defence, while at the same time being insecure about all the things she can't do. Gets as much character exploration as Meer, i.e. a lot.
Oddly unsexy, though. Has enormous breasts, but horses are not seductive.
4. SUE (slime) - the most unusual girl, perhaps because this is the only species that was made up for this manga. Well, unless she was inspired by Green Slime in AD&D. Sue's basically non-sentient. She mimics. In later volumes, she's honed this to the point of being able to speak and hold conversations, so it seems likely that she's developed a kind of intelligence, but no one's yet subjected her to a Turing Test. It's also dangerous to dehydrate her, which will make her attack people for their bodily fluids. (In other words, forcible lesbian deflowering.)
Has some peculiar powers. Often comes across as childlike, for obvious reasons, and hangs out with Papi a lot. Can absorb and digest almost anything, including toxins and industrial fertilisers, but might then suffer a temporary personality change.
5. MERON LERELEI (mermaid) - a masochist and tragedy queen. She wants to be like Hans Christian Andersen's The Little Mermaid, which means she doesn't want to marry Kimihito. She wants a tragic doomed love story that ends in her death. Otherwise, she's a nice girl.
6. RACHNERA (arachne) - the only girl with as much character development as the Serious Two. She's damaged. Underneath her shell, she's a wounded idealist who might possibly be rejecting everyone as a self-punishing test of whether or not she's been accepted. At the same time, though, she's just flat-out evil. The other girls are kind and law-abiding. Rachnera couldn't care less about the interspecies laws and would probably torture, kill and/or eat anyone she didn't like. Now factor in her sadism, a spider body to give you nightmares and a fondness for hiding in the shadows (or on the ceiling) to ambush her human prey.
She's arguably the manga's most interesting character.
7. LALA (dullahan) - even darker than Rachnera. She's Death, basically. She even freaks out the other girls with her detachable head and the nearest she gets to romantic sensibilities is, "But I am a Dullahan. When you die, your soul is mine." Said with her severed head under her arm, naturally.
Those are just Kimihito's housemates, by the way. There are other characters too, obviously. Kimihito's cultural exchange coordinator, Ms Smith, is a lazy, manipulative incompetent who exploits him as a trouble sink. (She's basically a Man in Black, but female.) She also has an extraspecies assault squad called MON, for those occasions when monsters are causing trouble and it would be illegal to send humans to deal with them. These are a genki zombie, a girlie but insanely huge oni, a surprisingly hot cyclops and an underused shapeshifter. (Is it just me, or are cyclops girls sexy? See also another recent monster girl manga, Nurse Hitomi's Monster Infirmary.)
I enjoyed it. I'll be watching the animated adaptation when it comes out later this year and I'll probably be returning to the manga in a few years' time after some more volumes have come out. However you will need high tolerance levels to get through it, both for its contrived worldbuilding (e.g. the improbably female cast) and its porn-a-licious events. Occasionally it's distasteful, e.g. close-ups of Papi's bottom. Under no circumstances to be read at work.
"Everyone else has... well, how do I put it? They all really seem to communicate how they feel through life-threatening injuries."