Makoto is an obsessive siscon that easily gets furious if there are other men near his younger sister. Kuusuke has an inferiority complex directed at his younger brother with psychic abilities, with whom he fails to beat everytime in every game.
Both Makoto and Kuusuke are shown to be inferior to their siblings and there's many parallels between them, just like between Kokomi and Kusuo.
Makoto is a famous actor under the alias Mugami Tooru, meanwhile Kuusuke is more known in academic circles in England where he went to college at 14.
While they don't ever interact in any of the manga chapters, they share other minor appearances in a bonus illustration, merch, anime openings etc.
The mystery surrounding those guy's view on eachother lets our imagination run wild What would they be like together? Maybe toxic and destructive, or the sweetest couple the world has ever seen?
Maybe you want them to heal their issues? Or make them worse? Take your pick.
There's a lot space to speculate and we can't have a single right answer to our questions based on their one illustration of them smiling at eachother alone.
Let's not forget their siblings! What would they be like when Kokomi or Kusuo were around? I think it'd be hilarious to see.