X-Men: First Class Head Canon

In one of those rare times of truce, when they had very little and very gray hair left between each other, Erik convinced Charles he deserved a special birthday gift. So he took him out in the middle of the night and he brought him to a local fair. Everything was silent and dark, but he flicked his hand and the ferry wheel came to life.
They jumped on it like excited kids and rode the wheel for about an hour.
And, despite all the differences and all the fights, for that hour it felt like the world was for them only to see and to live.

Sometimes, when Erik is alone at night, he closes his eyes and thinks about Charles. He tries to remember his face, his smile, the feel of the tweed jackets under his hands, but he rarely can. He’s a memory so far away from him that he cannot reach it, even if it’s the thing he wants the most.
So he thinks about going to Raven, to beg her to turn into him. But he never does. That would be a lie, and lies are not accepted.

One time Raven tried turning into Charles to see if her brother and Erik were talking about something the kids didn’t know, those times (a lot of times) when they were alone. She went to Erik looking like Charles, imitating her brother like only a sister perfectly can.
Erik then smiled at her and called her out on it. She asked him how he knew and he told her that, when he was around Charles, he could always feel his mind in the back of his own, even when he wasn’t using his abilities.

The actual reason why Erik put on the helmet wasn’t because he was afraid Charles could read his mind. It was because he was afraid his own thoughts would be too loud for Charles not to hear them shouting.

The first time Erik mockingly called Charles professor it was after a lost game of chess. Sometimes Charles tries to remember the game, the evening or even the day it happened, but for some reason, he can only remember the feeling in his gut when he saw Erik smiling playfully at him.

After his youth in Germany, Erik always ate alone.
One evening he sat with Charles at the dining table in the CIA facility and ate in silence: Charles observing him like he was trying to pull something out of his head without having to use his abilities; Erik eating quickly, devouring his food like someone was going to steal it from his plate, or worse, like food was a loss of precious time.
Since then, there had been many lunches and dinners with Charles, and food became something besides a waste of time. Until Charles wasn’t there with him anymore and he started eating alone again.

Apart from that one time on the beach, Charles never blamed Erik again for the accident. He didn’t need to try and read his mind, helmet or not he saw him blame himself enough for the both of them.

Erik didn’t know about what happened to Charles on the beach until a couple of days later, when Emma decided it was time to tell him. He thought about going back to Charles, but he never could convince himself to really do it. Instead he bought himself a chessboard and placed it in his room, in front of his bed, where he could move just one piece a day to see how long it took him to finish a game without his best opponent.

Erik never threw away the bullet that he pulled out of Charles’ back that day on the beach. He always carried it in his pocket instead of the coin. The coin was a reminder of the pain someone else inflicted on him, a thing to be used for vengeance. The bullet is a reminder of the pain he inflicted on someone else, on Charles. Sometimes he closes his fist around it and he swears he can feel it burn.