Gretto Avaro (グレット・アヴァーロ, sometimes romanized as "Gerd") was Maiza Avaro's younger brother and a passenger aboard the Advenna Avis in 1711. He was romantically involved with Sylvie Lumiere. Maiza imparted half of his knowledge of how to make the Elixir of Life to him, after which he was devoured by Szilard Quates. Little is known about his personality, but Sylvie claims Firo reminds her of him.
In 1705 at the epilogue he goes to greet his brother who at that time was going by the alias "Aile". In the novel Children of Bottle, Sylvie recalls that sometime before the summoning of the demon she confided her dream of obtaining "eternal beauty" to Gretto, who told her that "appearances don't matter" and that he found her attractive as she was. During the same conversation, he asked her to marry him, and promised that they would live together for eternity if they ever gained immortality. This suggests that his attitude towards immortality is quite different in the novels than it is in the anime.
Gretto is present for the summoning ritual on board the Advenna Avis, and is given half of the knowledge of the recipe for the Elixir by Maiza. It is not revealed how he felt about this. The following night, while the Avaro brothers are sleeping, Szilard sneaks into their room. He intends to devour Maiza, but can't tell the two apart in the darkness and accidentally eats Gretto instead. While this is happening, Maiza wakes up and manages to escape.
Gretto is first seen having a private conversation with Sylvie about Maiza's plans to summon a demon. He expresses fear at the prospect and doubt that humans really need to live forever, and declares that as long as he can spend his life with Sylvie he doesn't mind if his lifespan is finite. This is a contrast with the novels, where Sylvie's account suggests that he wanted to live forever as long as she was by his side. Despite his doubts, Gretto stands by Maiza's side during the summoning ritual, and drinks the Elixir, albeit reluctantly. The following day, when Maiza announces his decision to keep the knowledge of how to create the Elixir to himself alone, Gretto is the first to declare his support for the decision. That evening, Maiza confides his doubts regarding the future of the immortals to Gretto, who seems unsure how to help him. Maiza gives Gretto half the knowledge of how to make the Elixir, explaining that he wants a trustworthy person to help him find worthy candidates for immortality and to carry the knowledge if Maiza himself is ever eaten. Gretto tries to refuse, saying that the responsibility is too much for him, but Maiza insists that there is no one else he can trust. Unbeknownst to either of them, Szilard is listening at the door. Sometime after Maiza leaves the room, Szilard arrives and devours Gretto despite his terrified protests, saying that this is Gretto's "punishment" for allowing Maiza to "play favorites" with the knowledge.
- In the original Japanese, Gretto calls out Sylvie's name before he dies, but the English dub cut this out for unknown reasons.