i don't know the full extent, but it's implied that flo has a twin brother named ebb, the orca is his nameless slave. however, flo's family has a tradition that requires one of the brother must be sacrificed to please the god as the other will bear witness. at the same time, one of the most faithful servant shall be chosen to kill the sacrificed one and inherit their name, becoming some sort of surrogate brother.
from what i gather is that
in flo's perspective, he's traumatised and furious to the orca because the orca forced him to not only see his brother died in the orca's hand, but also has to acknowledge and live with the orca as the new 'ebb' due to tradition. (that's why he said living with nightmare he can't escape, and also why he's getting irritated about praying)
in orca's perspective, he's been living as obedient and fearful slave his whole entire life, disobeying god's word is definitely big NO for him. so, he faithfully took upon the tradition despite knowing full well that flo will hate him forever. (that's why he just let flo snaps at him and even abuse him)
i don't know if this is all, but it's what i understand