What is the role of singer Johnny in the novel The Godfather?

On the one hand, even Hollywood will be influenced by the will of the godfather (contradictory to Ulcz). On the other hand, it shows the Sicilian tradition. You are my loyal godson, and I am your holy godfather. The truth of this novel is reflected in the respect and full expression of Sicilian tradition. In addition, starting from the original, Johnny is also a small role. Even if he is glamorous, he is also a doll in fate. He will suffer from illness and marriage. He is a typical representative of Sicilian American immigrants, and his struggle is also part of the godfather's concern for Sicilian immigrants.

I am a godfather fan, please give me the best.