Firmo is Jerônimo's rival in The Slum: a skilled and idle lathe operator, a former capoeira, Rita Baiana's lover and the violinist of the pagode, who wounds Jerônimo in the belly with a razor and is later killed by him in an ambush at Praia da Saudade.
The narrator introduces him precisely: "He was a lathe operator, a skilled and idle one; he earned a week's wages to spend in a day". He was born in Rio, served from the age of twelve to twenty in capoeira gangs and once decided elections. Physically, "he had no muscles, he had nerves". At the Sunday pagode, he plays the violin while Porfiro plays the cavaquinho, and their chorado silences Jerônimo's fado.
On the samba night, Jerônimo whispers a proposal to Rita, and Firmo challenges him. The narrator weighs the two men before the blow: Jerônimo is "the calm strength, the lead fist", and Firmo, "the nervous strength". Firmo wins the first round with his feet and his head, but Jerônimo runs for his club and subdues him with a blow to the head, and Firmo, to escape, wounds him in the belly with a razor. No one reports him, and he is banned from returning to the tenement.
He moves to Cabeça-de-Gato, where he becomes a gang chief and cools off his affair with Rita. When he learns that Jerônimo has left the hospital, jealousy grows in him "like the thirst of a wounded man". He is lured to the beach by a lie about Rita, held by two figures, disarmed by Pataca and beaten to death. The Cabeça-de-Gato gang invades São Romão to avenge him, and the battle ends only when the tenement catches fire.