One of the challenges that Rui had at the moment was that he didn't have the best compatibility for poison. He recalled when he tried creating a technique that involved poison but failed because his aptitude for poison techniques was not the best.
It was unfortunate, but Rui did not know what constituted poison aptitude. He had temporarily been resigned to this limitation but did not abandon the possibility of using poison techniques.
As someone who aimed to be able to adapt to defeat any foe in the world, he needed to have poison techniques. After all, there almost certainly was a Martial Artist out there who, perhaps, could only be taken down with poison.
In that case, Rui would be unable to adapt to them without a poison technique.
That was unacceptable to him.
Of course, Reaper's Dew was not a poison that Rui intended to use offensively. Rather, he intended to use it supplementarily.
