Ender is not like Valentine and he is not like Peter. Ender, or Andrew, is a combination of both. Valentine is a compassionate person. She will defend her little brother if necessary, but she does not have the terrible mean streak that defines Peter. Peter is mean, jealous, angry, and sadistic. He uses people's fears against them and tries to control through fear.
Andrew Wiggins, Ender, is a good mixture of both of his siblings. He is a third. The third child in a family is not acceptable in the society in which Ender's family lives. The world is being attacked by the "Buggers" and the military is looking for young people who can be trained to defeat this cunning enemy. When Peter doesn't qualify because he is too sadistic and Valentine doesn't qualify because she has too much compassion, the military and government give the Wiggins permission to have a third child in the hopes he will be able to be trained.
Ender demonstrates his "Peter" personality when he attacks Stilson for being a bully. He takes advantage of Stilson's being on the ground to beat him to death. He is determined to show the other children it would be against thier best interest to retaliate. Immediately following the beat down he gives his enemy, Stilson, he rounds the corner and cries for the need to hurt anyone in the first place. This is a definite indication of his Valentine side. When Ender arrives home after school, Peter is first to recognizes that Ender's monitor is missing. Peter takes the opportunity to be hurtful and mean while Valentine tells Ender she is sorry that he has had the Monitor removed; thinking he has failed.
"Ender is like a combination of the two: ruthless enough to earn total victory, but compassionate enough to hate the methods needed to gain it. He is also different from Peter in that he does not always feel the need to win. He only fights when forced to, and takes little pleasure in his victories. When Valentine explains to him how he can defeat his oldest nemesis, Ender replies: "You don't understand.... I don't want to beat Peter.... I want him to love me."