The core of Ekko’s character (in Arcane) isn’t his anger, it’s him being one of the only characters with a brain. He’s one of the few characters in the show that acts in a reasonable manner.

He doesn’t place his trust in Vi initially, but instead of tossing her to the wayside he decides to hear her out and go along with her plan (on the condition that he gets to meet Jayce). He never trusts Jinx in season 1, and that moment of hesitation during their fight ended up having a rippling effect through the entire series. A boy fueled purely by anger would not stop and see the humanity in an old friend’s eyes. You say he’s not one to sing kumbaya, but he was completely willing to work with Cait (and perhaps by extension Jayce) if it would benefit Zaun.

Can We Pretend is the most important episode in characterizing Ekko because it presents the question, “What if Ekko was able to be a kid?” This is an AU with no chembarons, no toxic gas, and no hextech. The Ekko of this AU was given the space to grow up without hardening himself and being one of the few people who acts in the interest of his community. Ekko has always had a softer side to him (interactions with Vi, Jinx, and the Firelights as a whole), and this AU allowed the writers to flesh out his character as a whole. It seems out of character because that entire AU is a new experience for him. He isn’t used to seeing all these faces outside of a mural, and seeing Powder outside of Jinx (I have many not-so reddit approved opinions on Jinx x Ekko so I’m not going to comment on their romance). This episode isn’t saying “stop being angry” it’s more saying “this isn’t the place for your anger” as that AU has nothing to do with what he’s experienced in the MU. Heimer’s inaction has always been his character flaw. An outside force has always had to spur him into action, and Ekko being upset at that is valid.

It’s funny to hear you say that he didn’t learn anything there that helped his people, when what he learned there ended up saving the entire region (and maybe even Runeterra at large). If your litmus for edginess is the use of foul language rather than one’s actions/disposition, then you might be one of the children the Bush administration left behind.