“Friends can betray you, but with an old enemy, you always know where you stand.”

“There's ways you can trust an enemy you can't always trust a friend. An enemy's never going to betray your trust.”

“Friends can betray you but books are always loyal”

“You cannot be betrayed by your enemies, only your friends.”

“Do not fear your enemies. The worst they can do is kill you. Do not fear friends. At worst, they may betray you. Fear those who do not care; they neither kill nor betray, but betrayal and murder exist because of their silent consent.”

“An enemy, Ender Wiggin," whispered the old man. "I am your enemy, the first one you've ever had who was smarter than you. There is no teacher but the enemy. No one but the enemy will tell you what the enemy is going to do. No one but the enemy will ever teach you how to destroy and conquer. Only the enemy shows you where you are weak. Only the enemy tells you where he is strong. And the rules of the game are what you can do to him and what you can stop him from doing to you. I am your enemy from now on. From now on I am your teacher.”