Marvel and Jonathan Hickman Teases Most Important Scene in X-Men History

Marvel posted a new photo on Twitter which calls this panel below “The Most Important Moment in X-Men History”.

Why is this scene the most important scene in X-Men History? We know that one of the people in these comic book panels are Charles Xavier, founder of the X-Men. But who is the other person?

There are some readers and fans who say that this is actually a youn Moira McTaggert while others think its Jean Grey. I still think this is a new character who House of X and Powers of X writer Jonathan Hickman will introduce as the book begins its publishing run.