Okay, let's get this out of the way at the start. This "one-shot" acts more like the introductory chapter to a series rather than a completed story in and of itself. This is going to come up multiple times in the review, and I may sound like a broken record, but it really does taint what otherwise might be a decent read.
The story is told from the perspective of Mikagami Eruma, who is the average grade school student loser that's used as a protagonist in many manga series. A new student has transferred to the school, nicknamed Satan from the first syllable of his two names, and everyone thinks he's a bit weird, especially since he confesses love to a new girl every single day. One day Mikagami decides to be nice to this new kid, and unwittingly gets dragged into a supernatural world. An entity that calls itself Cupid shows up and decides to help him get points with the girl he likes, but his advice isn't that good, and then Cupid attacks him and Satan saves him. We learn that Satan is part of an organization who's goal is to protect humans from malevolent Angels. Mikagami now continues his life knowing that Satan is much more than he seems, and that's where the manga ends.
There are a lot of dangling plot threads that aren't really even explored that much in this single chapter. We only get the basic premise, the surface of the world the author intended, and not enough to really be satisfied with anything. Also, the romance subplot is cut off right at the beginning, with the girl maybe having more to do with all this than it initially seemed. This is made all the more frustrating because there is just enough meat and intrigue to possibly make you interested and invested, but you are not going to get the resolution to anything, and since the story really isn't developed past this stage, all it really feels like is incomplete.
There are only three characters that are explored in any kind of detail, the perspective character, Satan, and the love interest. The problem is that they are all shells because they haven't had enough time to get into the intricacies of these three people. While there may be a good start to what could be fun characters to follow, you really don't get enough of a look at them to really make any sort of decision toward how compelling they are.
The art is the typical clean, polished shonen style. Character design is appealing, though it doesn't really stand out in any way.
So, yeah. I'm not going to say don't read it, but just know that you really aren't going to get much satisfaction or investment out of the story.