I have watched the anime years before trying the manga Kiseijuu. I enjoyed it and didn't specially plan on reading the manga.
It baffled me that such a popular work had no reviews on this site since such a long time, so here we go.

Plot/Story
The way IWAAKI Hitoshi wrote Kiseijuu is pretty interesting. There is a noticeable pattern between volumes :

the three chapters before the volume end are always used to build up the intrigue that will follow in the next volume.
The first half of the third last chapter usually brings closure to the plot we were in, and then the new intrigue starts in the later half of the chapter. Sometimes the conclusion is at the end of the chapter and the new plot starts in the following chapter.
It makes it so that, when you think the story is finished, there is more ! That's usually pretty unexpected and appreciable. And when the volume ends, you obviously want to know what will happen next, since you've seen the plot build up for two complete chapters already.

Interesting variations :
In volume five, the new plot starts only on the last chapter of the volume, making the gap between the two stories a bit more abrupt than before. This feeling is also amplified by the fact that we are reading from another character's pov from this chapter on.
Same thing in the third last chapter of volume seven : there is a change of character pov and storyline, creating the same writing effect that is making you dig into a new plot for two chapters until the volume ends.

The two main povs will meet in the third last chapter of volume eight and conclude the ongoing plot there, making this resolution even more powerful.

Characters
The best characters are those who can school mc, make him a better person. Obviously his parasite would be in this category, although it's a bit hard to judge it as a complete character, since we rarely get it thinking by itself and only mostly with mc.
Only the substitute teacher and the detective fall in this category, since they are the only ones protecting their family. All the other characters are aiming to kill parasites or kill humans and not to protect people or themselves. Mc is more often trying to kill parasites than trying to protect his loved ones too.
He usually manages to save them after he forgot they were in a dangerous position, in which he more often than not put them himself.

Drawing Style
The way mc's face is drawn so pristine, like he's beautifully shaved every morning when he wakes up, to differentiate him from other characters. This is also giving him an invaluable heroic look.
If he's already special with just the way he's drawn, that's a smart use of the art style.

The more pronounced use of shades to signify the change in environments is cool, it's even better that it's not used all the time.
Same thing with sharp gazes or any form of animosity that are shown using darker types of shades, which are also occasionally used to expose a villainous character, parasite or human, to the reader. It's pretty straightforward and brings tension just by drawing the characters.

Double pages are the most impressive points with fighting scenes. They have very good detail and usually portray a really important moment/turning point in the story, and never fail to be impressive, or even breathtaking at times.

On the other hand, faces can be a bit too angular sometimes and not seem very human. I guess it would make sense for parasites, but they are blending in with society and aren't recognised just by their face, so it can still be off-putting.

Enjoyment
I honestly though the story would end at volume eight with two and a half chapters left. This feeling subsisted until volume nine with two and a half chapters left, as I asked myself again, why doesn't this stop here ?
Then the last segment was good enough to completely overshadow the previously lacking part.
However, after this final book, there are no more volumes and new intrigues. So, the later half of the third last chapter is used as a finale.

And then, I'm here, asking myself again for the third time already : why the f*ck are there two more chapters ?
After reading them, they just feel like an epilogue, with a questionable or even bad first half and an intriguing and interesting later half, that concludes the series well, even though there were multiple instance when it could have been done, and it would probably have been for the better.

So in the end, I felt that the second to last scenario was dragging the story down and couldn't really make me go into the last act confidently.

I still think the story should have ended after the

sub teacher's death , if I were to prioritise my own enjoyment over author's chosen ending. The following was good, but worse than the rest imo.

Overall
The parasite mother is such a great villain without her even trying to be one, that she overshadows any other enemy parasite.

When she dies, the story should be getting closure, and it does, but author chose to continue writing another scenario.
What follows is only dealing with the aftermath of having wiped out the only reasonable villain and giving the unruly and dangerous enemy free rein to his actions. Which doesn't feel all that great, considering how impressive sub teacher and her death were.

The plot rating, character rating and my own enjoyment of this work take a hit from that, but it's mostly because mc isn't good enough to protect his loved ones that the character rating can't get any higher than this.

8.266/10