Tommy is a truly fascinating character. He brings forth the idea of duality of the novel, representing both destruction and repair. Before he and Nicole become lovers, he is a professional soldier who will fight for any country. He is a self-proclaimed killer – a destroyer. To get at Tommy, let’s look at his conversation with Rosemary at the party at Villa Diana:
"Home? I have no home. I am going to a war." […] What war?"
"What war? Any war. I haven’t seen a paper lately but I suppose there’s a war—there always is."
"Don’t you care what you fight for?"
"Not at all—so long as I’m well treated. When I’m in a rut I come to see the Divers, because then I know that in a few weeks I’ll want to go to war."
"You like the Divers," she reminded him.
"Of course—especially her—but they make me want to go to war."
Here, we can see the destruction aspect of Tommy, and later he even tells McKisco, "I’m a soldier. My business is to kill people." We can also see something that doesn’t seem extremely significant until later in the book (here’s where the repairing part comes in) – his love for Nicole. Apparently he loves her so much that seeing her with Dick makes him want to kill people.
Though we can make an argument for jealousy, his reaction to the Divers also stems heavily from a genuine concern for Nicole’s wellbeing, and a belief that Dick is hurting her. He tells Dick much later, "You don’t understand Nicole. You treat her always like a patient because she was once sick." But even at that dinner party, he seems to be the only one outside of the family and the doctors aware that Nicole is mentally ill. He has a nurturing side, but he doesn’t seem to have a non-destructive weapon to use to help Nicole. Until later that is.
When Nicole decides to become his lover, the novel suggests that this is just what she needs to separate herself psychologically from Dick and to stop thinking that Dick and only Dick can make her healthy and whole. Tommy becomes an agent of Nicole’s repair.
We could also argue that he’s an agent of the destruction of the Diver family. And that’s the whole point. Tommy represents both destruction and repair. Perhaps most soldiers do, come to think of it. The most basic theory behind warfare is that one uses violence and destruction to bring peace. What do you think?