Reyna is meant for players who trust their mechanical aim above everything else. Unlike other agents who rely on complicated utility, Reyna’s kit is fueled by her own kills. Each kill she gets drops a soul orb that allows her to either dismiss (which means she is unable to get hit) to safety or devour to heal back to nearly full health instantly. She is the perfect choice for aggressive entry fragging as long as you can get the opening kill to kickstart her abilities. She also has a leer eye, which blinds the enemy. This aids in her fragging, allowing her to get kills easier.
ScreaM is one of those players whose aim surpasses all else. He is able to take on fights and be confident that he will win those fights. He relys on his aim, and Reyna follows behind to help him heal up or dismiss away to get to safety. Regardless of what he does, he knows he will be able to get the shot off before his enemy with pinpoint accuracy. So, the player is what makes Reyna so dangerous.