Lol I know
While I do agree with you, you also have to remember that if Jun wasn't throwing monkey wrenches around like party favors, Kou and Touya's plans would basically go off without so much as a hitch - hich would provide very little satisfaction after a bit.
The other alternative is the author pulls something to throw off Kou and Touya's plans out off thin air.
(Diablous Ex Machina for you tropers) And that sort of senario is REALLY hard to write in ANY medium, and if constantly used, it would become really annoying to read really quick
So Jun antics provide a vital narrative element: A justifiable source of problems so Kou and Touya actually have to legitimately work to get their plans to work - which is a scenario that is more satisfying for most readers
I know this is late but... Someone needs to shut Kouichi's brother up! Jun keeps going around trying to find out Kou's secrets because he likes him, but then divulges important information out to random people like the Himis. Jun, must we remind you what the definition of the word "secret" is or do we need to punch you in the throat to keep you quiet!?!