Curious and imaginative manga, falls a bit short due to inconclusive ending but it's short and intriguing read nonetheless that's easy to recommend to people looking for something more strange and unique in their fiction.

Story:
Alien 9's most remarkable point for me is the world Tomizawa manages to craft over mere 30 well paced chapters. Right off the bat it starts with a class choosing a new member for the alien party, a group of 6th graders tasked with taking care of regular alien threat besieging the school and it only gets weirder from there.

I can't say the plot is as intriguing as it doesn't really go anywhere, near the end there's a reveal that contextualizes what was happening prior and could've taken manga in a whole new direction but the whole thing ends shortly after without wrapping things up in a way that would be satisfying. It's the "but the journey goes on" type of deal before they journey really got to unfold. As such I treat this mostly just as short glimpse into bizarre sci-fi world.

Characters are decent but too remarkable, main protagonist Yuri is a useless crybaby that remains such throughout the entire manga, she doesn't really change or learn anything, her main flaw remains until the end. Perhaps the only thing of note happening is her getting attached to her friend Kumi but it's mostly there to justify a plot point later on and has little effect on her character interactions or decisions she makes. Overall she's more of a blank slate plot device than a character.
Kumi and Kasumi are a bit more interesting cause roughly 2/3 of the way through they undergo some major changes revealing which would spoil too much but it adds another trait to each of them, changes the way they behave and overall is an interesting mechanic even if it's a pretty blatant and easy way to force character progression it works.

There are some themes of loneliness, abandonment, dependence, coexisting and puberty but all I can say is that they are kinda there, none of them are explored to any meaningful extent and are just thrown in there. You might get something out of them as like most things it's up for interpretation but there's nothing substantial in here to make any conclusion more than just guesswork or a stretch connected with vague symbolism.

Art:
While it's not quite refined or consistent with proportions and basics it's still quite imaginative. Character designs are cute and distinct despite some degree of same face going on. There's no shortage of detail when it would be needed aside from occasional backgrounds. One thing that I want to praise more than anything is how amazing alien designs are, they're unique, memorable and bizarre. There's a sense of mystery to each of them, you can't gather what they will be just from looking and it does feel like those things came from another planet even if some look like a twisted combination of animals you might know.

Overall it doesn't have that good of a narrative but everything else in there is worth taking a look at. It's a fun experience regardless.