Responses
Well, it is interesting how they are recognized as twins and Rin is seen as Satan's son and referred to as such but Yukio isn't. If they are twins shouldn't Satan also be his father?
Yukio isn't really called that because no one sees the resemblance between Satan and him. That's why he takes meds all the time to supress his power because he's not honest like Rin. But when he fought Toudou his powers came out because he was extremely stressed.
The truth behind his birth??? interesting.... then maybe they are not really twins??? maybe Yukio was just created as storage to Rin's power by Mephisto??? Just a conclusion (▰˘◡˘▰)