Nathan Christopher Summers is Rachel's brother, sort of. In her timeline, she had no brother. She was the first born and only child of Jean and Scott Summers. Now she finds here in this timeline that Nathan is Scott's first born and only child. Sheesh, talk about confusing. Not only that, but Nathan's mother is a clone of Jean Grey. Then again, so was Rachel's mother. What a family!
Nathan was sent to the future as a baby to save his life because the evil Apocalypse had infected him with a techno-organic virus that could not be cured with current technology. The group that saved him was known as the Clan Askani (meaning "outsiders"). Interestingly enough, this group was started by an older Rachel Summers who had gotten thrown into that future timeline.
Nathan grew up in the future with his life's mission being the destruction of Apocalypse who was still alive since he is apparently immortal.
When Nathan was young he was raised by 2 people called Redd and Slymm who he later found out were Scott and Jean Summers but in different bodies. This was all done by the older version of Rachel called the Mother Askani in order to give them a chance to raise him as they should have done in their own time and to teach him to better control his powers as well as protect him.
As an adult, Nathan, also known as Cable, traveled back into the past to stop Apocalypse from ever getting into a position where he could rule the world and make it a living hell as he did in the future.
Well Apocalypse was stopped except it wasn't Nathan that did it, it was his father Scott Summers also known as Cyclops. He sacrificed his life and literally merged physically with Apocalypse so that he couldn't do it with his intended victim, Nate Grey (a Nathan from another timeline.)
Now Cable is about to find out that trying to stop Apocalypse was only the beginning of his "mission" as he has had visions of 3 "witches" who seem to be guiding him toward other situations where he can make a difference.
He has recently joined up with the X-Men in honor of of father. Currently he is trying to rescue Rachel from the clutches of a villain called Gaunt, supposedly one of the most evil men to ever live.
Click here to see Nathan as a baby with his family