throughout the story, the demon falls more and more for the girl and made more sacrifice to help her
she steadily gain knowledge and mana and is unclear whether she fall for the demon
demon sacrifice too much power and can no longer do the ritual to help her
she went to another demon instead
she did the ritual while her love for sister is true, and become angel instead
turns out the criterion for promotion is ritual + mana + true love = angel, fake love becomes demon
demon ask angel-girl if she reciprocate, she said no
demon is sad
angel-girl give demon some mana, and demon promoted
turns out the unlock criterion is love someone, not getting loved, something he did not understand, so he can never unlock
both become angels
happy end~~~
...
turns out angel doesnt have to be nice, girl became un-nice throughout the story
girl is prohibited to help sister with angel power
girl ignores prohibition and help anyway
becomes demon
girl - demon
sister - live human
demon - angel
happy end 2~~~