The Light Novel series is the source material for the adaptation of the Anime and Manga series. The information and terminology on the article will be based on the source material rather than the adaptions.
Arno (アルノー Arunoo) was a grey priest who served as Ferdinand's head attendant.
Arno appears calm and serious outwardly, like all of the High Priest's attendants. However, internally, he is motivated by spite. He feels no sympathy towards people he feels behave improperly, and celebrates when they suffer.
Before becoming one of Ferdinand's attendants, Arno was an attendant of the previous director of the orphanage, Sister Margaret, alongside Fran. Arno loved Sister Margaret, and became jealous of Fran when Sister Margaret forced herself upon him. His jealousy turned to fury after Margaret's suicide, when he learned Fran felt relieved by her death.
Much like Fran, he took his responsibilities seriously and carried his orders out with efficiency being a top priority. While he never openly shows distaste for Fran, he secretly enjoys seeing Fran suffer in uncomfortable situations.
Arno was the one who made the suggestion that Myne could use the orphanage director's chambers as her own. His motivation for this was to force Fran back to the chambers, to make him remember the trauma he suffered there.
By telling the Temple Hierarchy#High Bishop's attendants that Ferdinand was unavailable, Arno indirectly helped to the attack on the temple. The High Bishop believed that both Ferdinand and the Sylvester were away, so he, along with Count Bindewald, plotted to attack and capture Myne inside the temple. However, Ferdinand was in the temple. Arno claimed he was not available because Ferdinand was in his hidden chambers, and did not want to be disturbed. When Fran and Myne come seeking Ferdinand's protection, Arno repeats his claim that the High Priest is unavailable. While others believe Arno is inflexible, in reality, he is acting out of malice towards both High Bishop and Fran.
After the attack in the temple, Arno told Ferdinand about Fran's traumatic history with Sister Margaret. Ferdinand, displeased at Arno's actions, had Arno killed. While both Fran and Ferdinand used euphemisms, and did not directly state that Arno is dead, it is later revealed that "climbing the towering stairway" is a common term for death.