Rin is generic but to give credit where its due he stands out due to his demony junk and his sword. It's not much, but it's enough to make him a recognisable character. It's like with the main dude from Seraph of the End. He isn't really anything more than your typical spiky black-haired cipher, design-wise, but nearly all promotional materials focus on that old-school uniform he wears. it sticks in the mind. It's all about quirks and accessorising, mostly.
Not that that leads to success (Lord knows a well-designed main character didn't save Killco-san over in WSJ), but it goes a *long* way