In The Things They Carried, what are the main character's traits, strengths, or weaknesses? As the story progresses, to what degree do his/her traits become more or less prominent, and how does that work out for the character (does it have good effects or bad effects)?

The main character in The Things They Carried, Tim O'Brien, has the strength of observation and of empathy with the soldiers around him. His weakness is that he joined the army out of fear of seeming like a coward. Over time, he develops greater and greater strength as a writer by observing the men around him--soldiers such as Jimmy Cross, who loves a girl named Martha back home who can never love him back. Tim O'Brien (the author) says of Tim O'Brien (the main character): 




"I feel guilty sometimes. Forty-three years old and I'm still writing war stories. My daughter Kathleen tells me it's an obsession, that I should write about a little girl who finds a million dollars and spends it all on a Shetland pony. In a way, I guess, she's right: I should forget it. But the thing about remembering is that you don't forget."

Therefore, Tim O'Brien's strengths as a writer and observer become strengthened over time as he lives through Vietnam and then spends the years after the war writing about what he witnessed and experienced. His weaknesses diminish, as it's clear that both fighting in the war and writing about it require courage (and he is therefore not a coward).
His writing has good effects, as it helps him make sense of his past. As he writes in the story "The Rainy River" of the shame he's had to live with for years, "by putting the facts down on paper, I'm hoping to relieve at least some of the pressure on my dreams." In other words, writing is cathartic for him and helps him shake some of the demons that trouble him, and writing also helps him remember and honor the soldiers who fought with him.