A character analysis succeeds best when the thesis ties to a strong purpose, such as a theme of the story.  In analyzing Charley (in all the ways the other responder describes), consider what these particular aspects of his character serve in relation to one theme of the story.  I quote here from eNotes on the story:  the author " has a particular point he wishes to make about the dehumanizing process of warfare on those who are in the middle of combat. There seem to be two purposes in this: One is to explain the state of mind of those with soldier's heart, to make it comprehensible to those who see the body but do not understand the worn out mind within. The second is to make a statement about war . . . ." What statement about war does Paulson make (its inhumanity, the paradox of war demanding duties, such as killing, that are immoral outside the context of war, and so on)?  What character traits (such as innocence, a sense of duty, impulsivity, etc) does the author give the protagonist to further this theme?  Finally, be careful to provide evidence, cite the evidence, and comment on that evidence as you develop the essay. What a great assignment to tackle today, when war is on the news every single time you turn on the television or radio!

A good answer to that question might be to analyze Charley's character in depth. Charley, the 15-year old protagonist of Soldier's Heart, is on the surface a good-natured yet naive boy who joins the Army for all the wrong reasons. 

Charkey enlists because he is told that it is a once in a lifetime opportunity worth taking sincethe war will only last "until fall." It can be said that Charley's rash decision to fight in the Civil War can be attributed to the primal intsinct of a boy wanting to prove himself a man. What better way that to fight for your country and what it stands for. Little does Charley know what he's up against.

Charley's metamorphosis happens when he witnesses battle firsthand. It transforms him from a sweet boy to a stoic young man unwilling to make any frienships for fear that that soldier will too perish as others have.