As soon as Calli's flying boat disappeared in the sky, William flew to where Erinys' flying boat was anchored.
He had left Celine in her care because he was confident that no one in the Underworld would dare attack a Ferryman who was responsible for ferrying the souls of the dead.
Also, he didn't want to take the pregnant Elf with him to harass Felix's soul because it might cause her unnecessary stress, which was bad for the baby.
When he arrived at the flying ship, he saw Erinys sitting on the crow nest of her flying boat, dangling her feet as if she was deep in thought.
The boat of the Ferryman didn't have this feature, but she had modified her own boat using her Hell Credits because she was inspired by the movie, Pirates of the Carry-bee-an.
William suddenly felt like teasing the little girl and sneakily hovered behind her. He then lowered his head and said "Boo!"
The Half-ling almost fell off her look-out-point due to being startled.