The most interesting relationship in the whole series, for me, is not the one between Charlie and her mother Rachel or the one between Tom Neville and his son Jason. It is the relationship between Miles Matheson and Sebastian Monroe.
They were best of friends before the blackout and knew each other from a young age. They kept close after the blackout and worked very hard to found the Monroe Republic and in the process, saving each others lives. They were closer than family.
Then Monroe's slow descent into paranoia and Miles eventual disgust with his friend's resulting behavior caused them to go their separate way. Even when Miles is repulsed by Monroe's actions and recognizes that Monroe needs to be stopped, he still cannot bring himself to kill Monroe when he had the chances. Miles still considers him a brother.
It is a great yet twisted Bromance for me.