The characters are all fun and likable and have sympathetic backstories that do a good job at making you root for them. The story, in all honesty, is pretty basic but a series like this doesn't really need some super complex plot or anything. That being said, there have definitely been some lorebait hints of something deeper going on in terms of plot, but overall the series typically keeps its focus on "assassin kindergarten teachers fighting wacky assassins."
But what really makes this series stand out in my eyes is how, in between all the over the top comedy, it's VERY good at making you emotional when you least expect it. I'm gonna be vague about this so as to not spoil anything, but there was a string of chapters recently that actually made me cry, and I don't exactly cry easily from fiction (If you're caught up with this series, you know what I'm referring to here). Again, Kindergarten Wars IS a silly comedy, but part of what makes the series so good in my opinion is that it knows how to use the overall silly tone as a foundation to juxtapose darker, more tear-jerking moments. Both the silly comedy and the darker moments stand out due to juxtaposing each other, which I'm honestly all for.
So overall, really solid manga. I think out of all the "Assassin/spy SoL" series right now (I think that can be considered a subgenre at this point lol), this one is definitely my favorite. I recommend giving this one a try if you haven't.
