It does this excellently. The show takes you on a dramatic ride through the twists and turns of their relationship over a period of about 2 years. The main characters, Kaoru, an introverted smart kid and Sentaro, an extroverted kid initially perceived as a bully by his peers, are two dynamic and well fleshed out characters, both with their own strengths and flaws that the other happens to account for perfectly. Sure, they both have basis in common tropes, but that does not discount their strength as characters, not even a bit.
You read that correctly: this is a high school anime focused not on cringeworthy high school romance, but on the developments of a sibling-like relationship between two dudes. It's a refreshing change.
There is a 3rd main character, Ritsuko. She is not nearly as fleshed out as the two main characters, but she does a lot to drive the plot forward, as she's Sentaro's lifelong best friend and Kaoru's crush. But her relationship with the two of them is mostly in the vein of a friendly trio, aside from a few episodes where it's primarily used as a plot device to advance the dynamic between Sentaro and Kaoru, and their romantic relationship basically does not progress at all for the entire series. They never even KISS.
For example, in the final episode, there is a shocking scene between Kaoru and Ritsuko, but the purpose of this scene was not to say anything about their relationship - it was a result of Kaoru trying his damndest not to reveal how much he missed Sentaro.
It is very much a one way dynamic between Ritsuko and Kaoru, which never develops into anything resembling an actual relationship - and that's OKAY, because the story isn't about their relationship! It's about Kaoru and Sentaro!
My one critique of Kids on the Slope is that Ritsuko's character is not fleshed out nearly enough for how prominent of a role she plays. We know more about Brother Jin, a side character, than we do about her.
But all in all, this is an excellent drama with some excellent animation and excellent music. You'll laugh, cry, cheer, and contemplate your own teenage years, and what else do you want from a drama? If you can stomach a high school anime, watch it.
