I've been into Ender's Game for some time, and have always wondered about his MBTI type.
To me, he's always seemed like an INTP, but with a little bit of INTJ thrown in.
Yeah... I'm gonna need to back this up, as a lot of people are getting the feeling that he's I*F* or INTJ.
Introverted: This one's doubtless. He makes- and needs- few close friends. He does most of his thinking on his own. He only treats one person, Bean, as a confidante. And then there's a sentence that, in my opinion, seals it: in Ender's Shadow, page 366, Chapter 18 (directly after he kills Bonzo) that says " and [Hot Soup] said, 'Let him be alone. Can't you see he wants to be alone?"
Intuitive: He regularly performs feats of the latter that would be impossible without Intuitive being his stronger trait, such as knowing that the Formics would attempt to cut off their retreat rather than directly assaulting their fleet, knowing that Bonzo's head would be exactly where it was before he smashed Bonzo's nose in, et cetera.
Thinking: He approaches combat with such a logical and unfeeling mindset that it completely and utterly rules out Feeling being his dominant trait. His empathy is more a result of his Intuitive trait, I think.
Prospecting/ Perceiving: This is probably the shakiest one. In fact, I have begun to struggle myself with it. However, he amount of creativity he shows, his willingness to adopt unconventional tactics, his eventual "screw it" attitude that ironically led to his victory over the Formics, and most of all his ability to improvise- these together show, in my opinion, that he is an INTP.
I could see him being somewhere in the 60%-40% range when it comes to Perceiving- Judging; in my opinion, his ability to improvise pushes him over to the Perceiving section. But I'm open to debate.
Tool used for assistance in arriving at my conclusion: Free personality test - short version | 16Personalities.