I just finished binge-reading the series last week, and the rivalry between those two is genuinely the engine of the whole story. It's not just two powerful people sniping at each other; it's a deep, institutional hatred built on centuries of ideological conflict between the human-led Kingdom and the demon realm. The Daughter of Evil, Clarith, is fighting for what she sees as justice and purification, while Miss Devil, Lucifenia, is defending her people's right to exist. Their clashes are brutal, but the writing spends a lot of time inside their heads, showing how they're both trapped by the roles history forced on them.
What struck me most was how their personal animosity evolves. Early on, it's pure vitriol—spells flying, armies clashing. Later, there are these moments of uneasy truce when a bigger threat emerges, and you can see them sizing each other up, almost recognizing a twisted reflection. The story doesn't rush to make them friends, though. The tension stays razor-sharp because their core goals are fundamentally incompatible, which feels more realistic than a lot of 'enemies to lovers' tropes. I kept reading because I needed to know which of their deeply held convictions would break first.