we see N&K only in their lives inside the apartment, the only fixed guest is DE, it's therefore easy to think that if there is a problem between N&K the "guilty" is of DE because there are no other subjects, but N&K doesn't only know DE in their lives, and they don't do all of their life inside the apartment, so... we know probably 1% of the people that they know
anyway i can't even be sure that DE has nothing to do with all quarrels and troubles but if Kira is really jealous of DE why is DE so often their guest? and why don't i remember a real quarrel between Kira and DE in these years? it seems to me that the relationship between Kira and DE is very good when they are together