Maybe someone can explain the ending for me? I'm too stupid to understand the ending. I mean why the heck junya even hire the esp just for that?? He doesn't even do anything and just going away, what the heck men.

From what I understood, Junya realised his brother's true role. It was not to make Inukai fall but to keep him in check, so he decides to do that.

To do that, he has to threaten him every time Inukai attempts to go crazy again. He built his own resources so that he would always be able to be a counterforce to Inukai, because killing him would only bring yet another leader such as him. The only option was to keep the current leader in check, so that a new one wouldn't appear.