I feel like Rui stands on business while everyone else just keeps making up stuff to cry about haha
As far as Natsuo and Rui are concerned they were casual partners prior and them not being related still stands. If they work out I doubt anyone will be crying a river after giving it any proper thought. Natsuo should just move on, it's not even so much using Rui- cause they're already a thing either way and if anything this is what she wants more than him so it should be chill.