The protagonist is a 28 year old, unemployed, alcoholic hottie named Narumi Kawase. For reasons unknown (no spoilers here, you'll have to find out yourself!), she's out of a job and is now scraping by as the assistant of an established manga artist. At first glance, this manga is seemingly another take on the harem genre, especially if you consider the amount of attention the deuteragonist Jun Yoshikawa gets. However, as the series progresses, you'll see a lot of the other characters being fleshed out and having their share in the spotlight, as well as see how their personal and romantic lives advance, which is a great thing since this series is very character driven.
The anime is a romantic comedy, slice of life type of deal. With the latter in mind, it's important to make sure your characters are fully developed and have a sense of groundedness about them. That is what "Kono Onee-san" does very well, and it's always a pleasure (and a riot) to see them interact with each other. Whether it be supporting each other through tough times, finding the solution to their problems, and seeing the sheer chaos that erupts from their misunderstandings, I did not have one boring moment while reading this series.
The artwork is a pleasure to look at (especially Narumi), and stays consistent throughout. There are some odd looking frames with some of the guys, but they're few and far in-between.
"Kono Onee-san" is a wonderful series that intrigued me from beginning to end, and it succeeded without being over-the-top like most romantic comedy manga. The only thing keeping me from rating this an overall 8 is, like a lot of niche manga I've read recently, its ending is hastily wrapped up and doesn't leave a satisfying conclusion. The author,Tsutomu Mutsuki, has stated that he wanted to have at least twenty more chapters, but unfortunately could not reach his quota. It's a shame that has to happen to such a good series like this one.
Personally, this manga is an 8 to me, but I'd recommend it as a 7 for everybody else. Enjoy!