This review will have spoilers, so be warned. If you want a quick opinion, this is a terrible mess of a series, so I wouldn't recommend it.

In Alien 9, three cute elementary school girls team up with cool frog aliens called Borgs to fight other aliens. Borgs can shoot drills out of their bodies, will do anything to protect their partner, and have cool, laid-back personalities. What could go wrong?

For reasons that were never adequately explained, elementary schools across Japan are training young girls to fight aliens, and it quickly becomes clear that there are almost no upsides to being in the Alien Club.

Story: The story should be the one appealing aspect of a series like this, but it managed to be written poorly enough to take all of the enjoyment out of it. There are lots of twists and turns within the short three volumes that make up Alien 9, but almost none of them are deeply explored, which left me confused about what was going on until well after the fact. Looking back on it, it all seems obvious, but that's only because I'm finished the series and have put some thought into it while writing this review.
In a longer manga, having information withheld from the reader is fine and can add to the impact a series delivers, but in a shorter series which makes no attempt to properly explain things, it takes away all the tension and makes it boring to read. I shouldn't have to trudge through an entire series just to understand why someone's hair is shaped like a drill.

Despite everything I just said, the series does improve near the end, and wraps up in a way that makes it feel a bit more passable. The final arc was alright and improved my opinion of a lot of the characters.

Art: With detailed depictions of aliens, and nicely drawn fight scenes, the art is easily the highlight of the series, and fits the tone very well. In less serious scenes, the girls are drawn more cutely, but it's clearly not the author's specialty, and shows it's age a lot.

Characters: In the cast of characters, there's the three main girls, the Borgs, and the teacher that advises the Alien Club. The three main girls all greatly change and develop over time, while still maintaining their friendship with each other. It's admirable that so much development was crammed into a short manga like this, but it definitely could've been handled a lot better. The Borgs don't change all that much, but I loved them from the beginning and they really stole the spotlight.

The teacher is one of the most hate-able characters I've ever seen in a manga. If her motivations had been thoroughly explained she might have been better, but as it stands, I can't help but think that the author had her do the worst possible thing she could at every turn, just to improve her status and protect her own hide.

Overall/Enjoyment: There was a lot of promising stuff in this series, but it quickly fell apart, and I was hard pressed to enjoy it or even continue reading it.