Reigen And Dimple Understand Each Other Better Than Anyone Else. They're The Two Adults In Mob's Life

Reigen and Dimple understand each other better than anyone else. They're the two adults in Mob's life who give him advice on how to use his powers, their viewpoints both self-serving but ultimately necessary. They're both excruciatingly lonely and doing their best to hide from it. They both think they can cover that loneliness up through the pursuit of worldly power before realizing that they only thing they really need to be happy is to be important to Mob. They both care about that kid more than their own lives. And it's only both of them, working together, that are able to get through to him when it matters most.

They have an easy camaraderie right from the start. They tease and banter with each other, but they're serious when the situation calls for it. Riegen is more honest with Dimple than he is with any other person. The Reigen manga confirms that post-series, Dimple consistently hangs out with Riegen even when Mob isn't around.

However you parse this relationship, it matters. They're important to each other. They're foils and narrative parallels and weird messy loving co-mentors of the most powerful kid in the world. They're friends.

Also, personally, I think it would be cool if they kissed.