Can someone correct these sentences for me?

Can someone check these sentences for me, please?

1. Jake Sully is the main hero in the story because he the one who solve the problem.

2. Jake takes part in a hero's journey by being a part of the departure, initiation, and the return.

3. He is an ex-Marine confined to a wheelchair and part of the Avatar program.

4.When his brother is killed in robbery, he decides to replace his twin brother in a mission on the distant world of Pandora.

5.Dr. Grace Augustine, the head of the Avatar program, considers Sully an insufficient replacement for his brother and demotion him to a bodyguard.

6. Grace is the mentor and help Jake transference into a Na’vi body.