Firo is one of the major characters that have a lot of relationships with the others, maybe he's the most important connection for many of them, especially for Vino, Ennis, Luck, Maiza, Isaac and Miria,
Czeslaw and Szilard.
Firo and Ennis' relation is something sweet and intense. They fell in love from their first meeting but because of the circumstances, they can't stay together.
Ennis is an homunculus that doesn't believe in life and in the others, but Firo teachs her to lives for herlself and don't kill anyone without a valid reason. Ennis is a sensible girl and Firo's words touch the deep of her heart. At the end Ennis decides to believe in him and reveals him the secret to eat
Szilard, her creator, and Firo and immortals' enemy. After it, Ennis begins a new life togheter with Firo and in the future they get married, becoming a real couple.
ISAAC & MIRIA
Isaac and Miria are two of the best friends of Firo. Their relation is very funny and sweet, Isaac and Miria love him as a brother and everytime they meet each other is a party for them!
Luck and Firo's relation is very important and close, because Luck is Firo's "compare" (big-brother of another family of Mafia, the Gandor's one) and when Luck is in difficult, Firo fights for him. Loyalty is something they feel in their soul and whatever thing would happen, one will always save the other one.
Maiza is Firo's mentor, the person who enshrines the secret of immortality and immotals. Firo is always by his side and even in difficult moments, he offers his help to him, thanking him to create the elixir of immortality, that he and the others drank.
CLAIRE / VINO
Claire is Firo's childhood friend and they're very close. Firo remains orphan when he was 9 and lives in street, when he meets Vino. Their relation is founded in reciprocal respect and faith, when someone of them necessities a hand, the other is ready to help him, even if it would cost his life.
Czeslaw is similar to Ennis. He thougth that he can't believe in anyone but after the meeting with Isaac and Miria he understood that not all the immortals are equal, he can believe in who wants to give to him a hand. At the end of the story he lives with Firo and Ennis like a younger brother (or a son, yes! XD).
Szilard is Firo's last enemy. He created Ennis and the homunculus and in order to posses the secret of immortality, he commits many bad actions and murders, and he never fouls his hands, leaving all to Ennis and his other men. Thanks to Ennis' help, Firo eats him and saves her, gaining his memories and informations too, together with the secret of immortality.