Because his most prominent features are loyalty and precision. Are silent suffering and suicidal tendencies so out if place for someone who has fallen out of society due to what he has experienced in the war? His face makes him an outcast, so he holds on to the only community he has left: Jimmy (and Angela). He left his family because things could not be the same again. He was changed. Now he has a picture-book paper family instead. An INFP could do all those things, too, it just was not my impression.
Also worth noting my guesses that opened this thread were based on info available I think partway through season one (it was written over a year ago). Obviously, a lot of those guesses have changed.