I have read the books several times over, and I even have spark notes on hand-
Ender is the youngest of the three Wiggin children. Ender has the compassion of Valentine, his older sister, but he also has the ruthlessness of Peter, his older brother. Ender does not wish to cause harm to anyone, yet when he is confronted with a pack of students led by the bully Stilson, Ender knows what he has to do. At age six he beats Stilson to death, although he does not know it at the time. Ender can be a killer like his brother. But Ender hates himself for that quality. Other people put him in situations where his negative side emerges, but Ender always wishes for events to be resolved without violence. His compassion is his strongest trait, and it binds people to him; his ruthlessness wins their unswerving faith in him as a commander.
Ender could be seen as a victim, a brilliant child manipulated by adults into playing a game he never wanted to play, but such an explanation is too simplistic. Ender is brilliant enough to understand the manipulation and knows that it is necessary. In the same way that Graff manipulates Ender, Ender manipulates Bean while he is commander of Dragon Army. Ender is a child in name only, and he represents the best that a human being can be, given the context. There is no part of him that desires control, and when he has it, he wishes only to avoid the abuses that he sees others commit. But Ender learns that in life we are sometimes forced to play games that we would rather not play, and that sometimes winning must be second to nothing else. Still, he manages to retain his humanity and his compassion wins out in the end, as he sets out to repay his debt to the buggers.
At first I thought he was INTP, but because of his selflessness and compassion, I'm inclined to think he's INFP. He even preserves the Bugger Queen out of sympathy.
Valentine is Ender's older sister, and she does what she can to protect him from Peter, their sadistic older brother. Valentine has much of Ender's compassion, but she does not have Peter's ruthlessness. However, as the story progresses, Valentine learns that the differences between the three siblings are not as great as they seem. She never ceases believing that Ender is the best of them, and she loves him unconditionally. In fact, her love helps save Ender when the pointlessness of Battle School threatens to break him. But as she begins working with Peter to transform the political system on earth, using her pseudonym Demosthenes, Valentine learns that there is a part of her that enjoys control.
Although she does not want to manipulate people for the sake of manipulation, like Peter does, Valentine knows that she is intelligent enough to make a major difference and sees no reason why she should not. However, she would never wish to do damage in her attempt to change the world. Valentine, like Ender, would never deliberately harm someone else. Unlike Ender, however, she falls prey to the irresistible urge to power. But that urge would never take her down Peter's road of power at any cost.
Valentine and Ender's older brother, Peter is completely lacking in the compassion that the other two have and has only ruthlessness in its place. He is a sadistic brother and it is unclear at many points whether or not he is considering killing Ender and Valentine. Peter is the mastermind behind the actions that he and Valentine take in world political affairs, and he is motivated by his thirst for power. His lack of compassion makes Peter a man devoid of humanity. He acts solely for his own benefit and, although he has a profound understanding of others, uses his knowledge only to probe their weaknesses to his own advantage.
Peter eventually gains power over the entire earth and becomes the Hegemon, the leader of the world. He is able to do so peacefully, although this is not necessary for him, it was merely the most expedient way to gain power. The difference between Peter and Ender, although it sometimes seems slight, is in reality a fundamental difference. Other people matter too much for Ender to do willingly endanger them, but Peter takes the opposite path. He is concerned only with himself, and although this leads him to be the ruler of all humans on earth, it makes Peter as far from human as possible. It is the nature of power, and the effect it has on people, that creates the resemblance between Peter and Ender.