I find it funny how each member of Promise fits into the other groups of EN pretty well.

IRyS => Myth
Both angel and demon, she's basically a mythical being even amongst mythical beings.

Kronii => Justice
She's the warden and justice are the jailers, LOL. Also, blue fits into their established colors well.

Bae => Advent
She is chaos and the members of advent were basically thrown in prison for obstructing order/creating chaos.