I don't remember if this has ever been discussed here: Was there a real life prototype for the Tom Hagen character?

I don't know if Puzo ever listed anyone as inspiration. To me the closest analogue would have been Murray Humphries.

I think that there were a number of non-Italians who gained prominence in organized crime and Puzo simply copied them. It added an "outsider" dimension to the novel and keeps the reader wondering why.

Meyer Lansky was close enough with Charlie Luciano to be considered his de facto consigliere. But, unlike Vito Corleone, Luciano never gave him the official title.

I thought that myself. The differences are substantial, but some of the attitudes towards Hagen (at least in the novel) were directed at Lansky when he was coming up with Luciano.

Frank DeSimone and Murray Humphreys.