Have you ever got bored on class? Probably yes. And you probably tried to do everything but listen to it, setting your own games with what you have in hands. That’s how Seki, the main character, passes his days on school. To minigolf with cracks on the table to a war game with pieces of chess. But what if someone on the class notice what are you doing and try to go along? And so you have Yokoi, the classmate who is the only one that noticed Seki and his games, and do some sort of narration for them, which are also the punchlines and where is the humor of the manga.
The relationship of the characters are also established this way. Seki is just the misterious boy who never talks and is a genius in doing improvised games. Yokoi just do the punchlines for herself and sometimes get so much on the games that she tries to prevent some of his doings. We also sometimes have another classmate which just misunderstands Yokoi actions and believes she likes Seki. As for their development, I don't really see any of the characters changing. And they don't need to, as the manga depends on them following their roles.
So, it's just a one gag for all the chapters. Sure will go down and became lame after some time. Well, I prefer to believe that the author has an idea to how long it will work and I don't see it lasting much longer, anyway. In the end, what matters is how your enjoyed what you could.