The N7 guy is an anti-hero. They will be your brother/sister, whichever you don't choose to play as. The sibling that has always been more successful while you are just a lowly explorer blah blah blah.
You two have a falling out because he/she wants the mission to go down a more bloody path. N7 guy leaves and forms his own sqaud. You run into them a few times while trying to redeem your sibling. In the end he/she helps you defeat the enemy and then disappears.
That's unrealistic - they aren't going to send a bunch of randoms/morons (generally unstable people!) to Andromeda and let them run wild, no:
There will be a chain of command IMHO so anybody not following orders has basically turned rogue! (Note: This is necessary, there isn't an abundance of people on this mission so everybody has to do their job at least till colonies have been established and some industry, military etc. has been established!)
ps: If he's your sibling he might be more successful at the start, but I bet that'll change soon (maybe he royally fucks up and that changes his perspective?)
pps: Still against this whole family BS (sorry: Family stories belong in soaps/sitcoms - not in good science fiction (at least not as part of the main story - so you can decide not to do the family related stuff...maybe you are estranged?)