O'Brien regretted to sacrifice Mayne to Zero, but did it nonetheless, believing Zero was more suitable to lead the Church in Battle of Divine Will.
He planed to unite the Four Kingdoms under the church, but he passes away before he can finish it. He arranged for Mayne to become the next pope officially and instructs him about everything he has to do as the new pope, but in secret he lets Mayne and Zero fight for the position of the pope, utilizing Zero's battle of dominance.
He had watched her grow up, and considered her an observant, clever and devout girl.
- The first time he hosted the ceremony, he was 45 years old. The strong smell of blood made him throw up directly on the stage, almost resulted in ruining a basin of blood. This mistake even got him whipped hard by the previous pope. After the punishment, he was ordered to get back to the stage and continue the ceremony.