- "Angelo, starting now, you and I are brothers. And we always will be. "
- ―Corteo to Angelo
Since they were both children, Angelo and Corteo has been best friends. He also got along with his family to the point where Luce and Angelo practically adopted him as their brother, he was also close to Testa, with whom he often discussed complex formulas with. He and Angelo seemed to have a special wave that they sued quite often with each other, signaling how close they are. After the death of Angelo's family, he flees to Corteo's house. There he is comforted before leaving town for 7 years.
Returning to his hometown seven years later, Avilio and Corteo are again reunited after years of estrangement. After hearing his request, Corteo relented and agreed to sell his alcohol in order to help Avilio despite his deep-seated dislike of the mafia.
(Anime spoiler alert) Corteo joined Fango in trying to take down Nero, believing that by doing so he can free Avilio from his revenge, but this ends in a very bloody ring. When the time where Avilio has been accused of helping Corteo escape the Vanetti's, Corteo sacrifices himself in order to save him.
In the novel, their relationship seems to be a lot closer than how they look like in the anime version. Barbero and Nero, for example, has noted how protective Avilio is when it comes to Corteo, stepping in front of him to shield him from view when someone is pressuring him and putting his life on the line against Fango to keep Corteo save. In the anime, the first thing we know about Angelo is that he has guts, enough to put out a candle with his hand despite not knowing beforehand that it's not hot. But in the novel, Avilio mused that the reason he can do that is because he fully trusted Corteo and believed in his intelligence to the point where he couldn't think that Corteo can be wrong about such a thing.
In one particular scene in the novel, Avilio dreamt about the time his family had picnic together and mused that the reason why he can have such a nice dream instead of the usual nightmare is because Corteo was there and it relaxed him.
When he was a child, Vincent betrayed his father's don and killed his brother and parents. Angelo managed to escape and now seeks revenge.
Nero Vanetti was present at the death of his parents and brother. Angelo now seeks revenge against him and his family.
He joined the Vanetti and became one of Nero's most trusted underling, to the point where Nero's right-hand man, Barbero, felt threatened by his presence. With the progression of the story, their relationship became even stronger to the point where Avilio declared himself Nero's brother after Frate's death. He did many things to make sure that Nero is safe, but his bitterness remained whenever he was reminded of the night seven years ago.
Despite him seemingly warming up to Nero after episode 4, in the novel that took Avilio's perspective and narration, it was written that he never changed his opinion of Nero, hinting that him warming up was just an act to be closer to Nero, and subsequently, Vincent.
Vanno Clemente was present at the death of his family's murder. Angelo now seeks revenge against him as well.
Fango is aligned with the Orco family, so he is an enemy of the Vanetti family. Angelo uses this to try and prove his alliance.
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