In the opening scene of the novel at Connie's wedding, Don C. is talking to Tom about "The Senator", who the Don himself advised not to attend, but who sent a gift of antique silver, "very valuable", that, as hagen points out, "The kids can sell for a grand at least."

Puzo then writes

Don Corleone did not hide his pleasure that so great a man as the Senator had shown him such respect. The Senator, like Luca Brasi, was one of the great stones in the Don's power structure.

Given the fact that he was placed on a par with Luca Brasi, and that a Senator (A Senator)) could certainly be more valuable to the family than a simple muscle man, (no matter how unique Luca Brasi may have been), don't you find it somewhat disappointing that no further mention was ever made of him?

If I were reading the book for the very first time, I would have certainly expected the character of the Senator to have been at least partially developed.

Well...maybe. The novel had enough action from that point on that I didn't miss the Senator. But he might have made a useful character in the section on Vito's rise to power.

I think that the Senator was mentioned for illustrative purposes. He was a building block for sure, but I think he was brought up to show the reader the difference between the Corleones and other families. They were not simply about muscle. They were not all about violence. Don Vito was smart enought to see that politicians and judges were just as valuable as the best enforcer.

True enough. But then we learn all about Luca Brasi, the stone who represents muscle and violence, and nothing about the Senator.

Maybe cause the Senator isn't a real friend, more a 'business-partner', while Vito considered Luca Brasi as a friend. They had know each other since the 'good days', and Luca wiped out single-handedly the Irish group that shot Vito.

Is there btw something in the book of when Luca and Vito met? Probably, but I'm too lazy too look it up.
It has to be after 1927, cause Luca wasn't in GFII.

Yeah it's been so long since I have read the book that I forget the details, but in a nutshell Luca was a right mess and threw a baby in a fire, the Don straightened him out and Luca's been eternally grateful ever since. Something along those lines anyway, I remember it being a bit strange

I think that the Senator was just supposed to be representative of the Don's political muscle. Luca Brasi, however, was there to illustrate how a man like Vito, who is portrayed as someone to like and admire, runs "that" side of the business. It is also to show how he gains the loyalty of such a man. The Senator? His loyalty is simply purchased, hence no interesting story.

Right, SB! In fact, I think the phrase used in the novel was something to the effect that the Senator, like Luca, was one of the Don's great building blocks. I think Puzo wrote it that way to make the point you made: that the Don's power rested on two kinds of muscle--violence and politics.
Enzo makes a good point about the Don regarding Luca as a "friend." In the movie, he even calls Luca "my most valued friend" [emphasis in original]. What does it say about the Don's "values" that he regards someone like Luca as his "most valued friend"? In the novel, Michael asks him about this, saying something like, "Why did you need Luca...an animal like that?" The Don replies to the effect that there are crazy people in the world who run around shouting "kill me, kill me." They can be used to advantage--which is what he did with Luca. That exchange put a bit more realistic spin on the Don's relationship with Luca.

And if I recall, Vito tells Michael that men like Luca don't fear death, in fact they beg to be killed. However, if you can become the person that they fear will kill them, then you will have their loyalty forever. That is how Michael gains the trust of Al Neri, who someone (Tom perhaps?) refers to as Michael's Luca. I believe that is why Luca is such a complex man and an interesting character. How many of us don't fear death?