For an anime about a boy who wants to become a novelist, Domestic Girlfriend committed a few sins on character development.
Beware, total spoilers ahead.
The premise is interesting: a teenager suffers from unrequited love for his teacher, tries to move on by having casual sex with a girl his age, only to find out that both girls are sibling and that they are going to live with him from now on. It creates a good tension. The fact that the MC's love is living with him makes it harder for him to move on, as now he shares intimacy with the woman; the same goes for the girl he had sex with, making his situation even more confusing and uncomfortable. As if that wasn't complicated enough, she is developing feelings towards him.
This set-up created an incredible environment for character development. But instead, the author just throws it out the window by turning the plot into a mess by the middle of the season.
Besides the premise, every story should have a clear goal for the MC. In this case, the goal is: he wants to date his teacher. So the ending of the anime should be about whether or not he achieved that. During the course of the story, he would have to go through a process of change, either of heart or character. That is how stories are made, and seeing how characters change while they struggle to achieve their objectives is what make stories interesting. Instead, in this anime the MC forces his way through his teacher's heart and achieves his goal by the end of episode 8. Leaving aside the fact that the MC went through no change, the result is that the last 4 episodes are boring, as the show tries hard to create some drama to compensate for a lack of objective.
Now, there are plenty of stories presenting characters that don't go through change, but they normally end badly for the MC. In this case, without change, the character would have ruined his and his lover's life. This is even acknowledged by Hina (the teacher) who says to Natsuo (MC) "are you sure you want to follow this path? To do so would mean committing social suicide, for both of us. Could you do that?". But instead, there are no consequences to their romance (other then they having to be separated, at least for a while). After they have been found out, the teacher transfers to another school, leaving Natsuo and her family behind without word. No charges, no scandal, no reproach – i.e. no social suicide.
In the end, Natsuo suffered a lot. As a consequence, he wrote a prize-winning novel. And he'll be banging the poor younger sister. All's well that ends well.
Because Hina left him, Natsuo was able to write the prize-winning novel. The last episode is not about how his world crumbled into pieces after a sordid affair, but about how the destruction of his world made him rise from the ashes as the phoenix, turning him into an artist. There's even the cheesy quote "Art is born of sadness and suffering". No matter how the author of the anime tries to mask this, the story shifted it's focus. If you break down the last episode into one sentence, it would be something like this: "and to achieve his goal of becoming a great novelist, Natsuo would have to suffer the loss of his first love". You see, even in the review it gets confusing, because that ending would only make sense if the initial goal was "Natsuo wants to become a novelist".
One might argue that Natsuo's goal was always to be a novelist, but that's not true. It is clear from the beginning that what he desires most is to be with Hina. In the first episodes, he is not portrayed as a bookworm/writer. It hints that he likes to write, but that's more a character trait than the objective of the story. Also, he only joins the literary club by episode 5. If he wanted so bad to be a novelist, if he loved so much reading/writing, why wasn't he in the club before? Regardless, even if this was his childhood dream, it was not the goal of this story.
There are many, many paths this anime could have followed that would have made it good. Unfortunately, it seems the author of the series has not been through enough "sadness and suffering", otherwise this might have been a prize-winning piece of art.
This anime had potential, but it was confused about it's own structure. The characters could have been interesting, but they lack development and growth. The plot could have been good, but it lacks consistency. In the end, although the first 4 episodes were engaging, this anime was a train wreck. It turned out to be boring and confusing, and left a bitter taste after it was finished.