There's nothing special to be seen in Nakaimo. The art and animation are sub-par at best, and most if not all of the character designs are nothing special, other than hair color and someone who wears a witch costume through a good portion of the series. Music did not stand out, nor make much impact on any dramatic scenes that deserved the attention. At the very least, the story itself is unique and a bit of a departure from the normal harem trope, but for reasons to be further explained in spoilers, it can't achieve its full potential of drama.
The biggest weakness of this anime is its story-telling and pacing, and the characters involved in it. A good harem would present its cast of characters, and put them through some emotional trial, and force them to go through dramatic character dynamics to help them grow and form more intense relationships with each other. Unfortunately, for Nakaimo, rather than having regular character development, it puts the viewer in the same position as the main character; unaware of his surroundings, and confused.
What makes this anime different from other harems is that instead of having to build relationships with multiple girls, our main character has to tread lightly and meticulously pick a suitor to marry, and is also wary of being lured in by his illegitimate little sister. So with this atmosphere, Shougo is constantly suspicious of the girls around him, hindering the relationships between them to grow. This is the exact opposite of what we need in a main character and his relationships with other girls.
-Minor Spoilers, but In-Depth review follows-
As if that itself wasn't enough of a problem, each girl has her own reason to actually go out of her way to fake being Shougo's little sister. This is the show's method of presenting trial for our main character, but in doing so it actually lowers the quality of the show. Being that it is a short series, each girl gets roughly 2-3 episodes to herself with her scheme of trying to pretend to be Shougo's little sister, and considering we don't actually find out who it was or why they did it until the end of their arc, there is little to no actual development of the character herself, nor any growth in the bond with Shougo.
When the final mystery of the anime is revealed to the audience and main character, the unfortunate truth is that the revealed character didn't receive the proper development over the show. Even the last few episodes that should be dedicated to this character were not, and further hindered the climax of the show. but it is fair to say that she receives the most attention and development compared to the other girls (which isn't saying much). Her and the 'side-kick' character that lives with Shougo, Mizutani Ikusu. She is probably the most interesting character as far as personality goes, for her upfront and bold appearance, while being naked for most of the show.
Nakaimo definitely has good grounds to be different, but unfortunately it's too short and too quick for it to get anywhere. It lacks the time and devotion needed to interesting characters, and hurts itself as a harem anime because of its unique plot and attempt to be bigger than it really is.
