Main hero of the book. We can describe him as a character that has a very dynamic role. He is the one who has changed a lot from the very beginning until the end of the story.
Frederic is a little bit aloof guy. And it is not a strange thing as his work is a stressful one. He is a young American ambulance driver with the Italian army during World War I. So, yes, he came through lots of hard things as well as he saw lots of blood and death.
By the way, he does not feel strongly about the cause and certainly is not out for glory. He turns from the horrors of war to a passionate, escapist love. Moreover, he met his military duties with quiet stoicism. He displays courage in battle, but his selfless motivations undermine all sense of glory and heroism, abstract terms for which Henry has little patience.
Important to mention that he really lacks passion in his life up to the time he meets the beautiful Catherine Barkley. It isn’t a secret that love cures and that all-consuming love helps to distract our hero from the brutality around him. Still, he is good at his job; a cool-headed, unselfish man who exercises grace under pressure when he is injured and when he must shoot a deserting engineering officer, Henry fulfills the code of the "Hemingway hero."
Finally, when he was on the crossroad, he made the right decision. He makes his "separate peace" when he decides that he no longer has any obligation to the army and that he is absolutely loyal to Catherine. It deserves to be granted, that is why we can hope that they could get married if only they were real.
Lieutenant Frederic Henry in the Essays