Fafnir and Quetzalcoatl (Lucoa) are the names of mythological beings. Fafnir was a dragon in Norse mythology, and Quetzalcoatl was from Aztec mythology (though she was described as a feathered serpent rather than a dragon).

This was the last show Yasuhiro Takemoto served as Series Director on before his death in 2019. He is still credited for season 2 in 2021.

Season two was announced in February 2019 and production was delayed by two years due to the July arson attack, in which the Kyoto Animation studio was set on fire, sadly killing 36 and injuring another 34 employees, including the series director, Yasuhiro Takemoto and art director, Mikiko Watanabe. The second season was thought to be canceled or at the very least would change studios before it was properly announced it would return with a set date for 2021, with Kyoto Animation returning to produce it.

Fafnir's (Daisuke Ono) human form resembles Sebastian Michaelis from Black Butler (2008). Both Fafnir and Sebastian are voiced by Ono.

Based on seinen manga written and illustrated by Cool Kyoushinja.