@Crimson_Blood V-poly, so Ash and Rin will both be in a relationship with MC and are sharing MC, while they both become really close and trusted friends and companions with each other. The only thing that prevents Rin and Ash from becoming friends currently in the story is because of the whole debacle with MC 
Ash has feelings for MC while Rin keeps being pushed to be with MC and seen as the best choice there is for MC and both of their families. Other than that, they both actually respect each other’s strengths. They also have pretty much the same moral compass since both of them are ruthless. And both of them plus MC have also known each other ever since they were all kids.