When Xavier heard the co-founder of his institutes oldest children was applying for a position, he had to accept. When they split up, he could not justify getting rid of them.
He teaches physical education, and has a reputation for favoritism. His rivalry with Scott also has lead to issues within the faculty.
He secretly manages an organization called the Brotherhood. He ensures that they stick together along with his sister. He tends to be over aggressive and actively tries to provoke a human response at times during missions.
He believes that humanity is to scared to actively resist the Brotherhood.