Is gramma

No... Remember that black haired boyo we saw on chapter 2 or 3? I think he was the brother of gramma

i think that's devil-san before becoming a demon

But he sees it in his dream and says whoever that kid is going to be a demon among demons... So.... And there is a part where gramma thinks to herself even if his brother became a devil if its that kind of devil and it says false below that we dont know if he even became a devil and if he did if he is a good devil

well yeah but unless he's a psychic i think that was just a flashback and he may have lost his memories of before he was turned, hence why he doesn't remember his sister, and he looks way to similar with the kid, like his right eye that is still normal and the cover of ch3, also knowing this type of manga i'm pretty sure that was what they were aludding to but i may be wrong idk

Maybe ur right but idk either rly xd

I agree. I think that's his memory of before he became a demon. He really did become the strongest devil of them all like he cursed he would ;-;
Too bad he can't remember his sister (the grandma). & I think the grandma realized it was him too because of the look she gave to his "human" form.