From The Desk of Liz Ensley:
Not that a lot of the character sheets I see cover this in much detail, but the way your main character interacts with others is important to a story. For example, what was Gandalf's relation to Sauron? What was Bilbo's relation to Sauron? What was the One Ring's relation to--well, you get what I mean. Believe it or not, you can consider the One Ring as a character, albeit a minor, yet powerful one. Sauron created it, and it subverted Gollum, and was on its way to doing the same to Bilbo, as evidenced in scenes in the book. That's how Bilbo and the Ring interacted, even when Bilbo Baggins was more of a background character.
To evidence it to yourself, write a sentence or paragraph about, say:
Harry Potter's relationship to Voldemort.
Harry Potter's relationship to [insert] Dursley [here].
Harry Potter's relationship to Albus Dubledore.
Harry Potter's relationship to Rubeus Hagrid.
Harry Potter's relation to Severus Snape.
Harry Potter's relationship to [insert] Weasley [here].
I could list more of them, but these are good ones to illustrate what I mean by what I think this means. The primary relationships we consider are usually family. We can start with the Dursley's, then.
Harry's relationship with Petunia Dursley. He is the son of her deceased sister, Lily. Petunia's relationship with Lily was not the best. As the books go on, we get the feeling that she was, perhaps, a bit jealous of Lily's being able to do magic, when she could not--maybe she and Lily were very close, before Lily received her Hogwart's acceptance letter. Perhaps she felt abandoned by Lily. What we do know is that Lily would tell her parents what she had been learning at Hogwarts.
Still, there are some unanswered questions there, too. Had they been close, and had Voldemort killed their parents (since the parents are not mentioned in the story--Harry's grandparents). Could Petunia have blamed Lily on some level for that, since Lily was a wizard? It would not even have had to have been Voldemort. From what we know of the Death Eaters, and of some other wizards who were sympathisers to the Death Eaters, well, you know the push for legalization of Muggle hunting that was put through. If we wanted to relate it to the main plot, perhaps they were killed when Pettigrew was trying to hide from Sirius Black and killed a whole bunch of Muggles after biting off his own finger. Black was a friend of Lily's and James. Actually, there are some minor things in the books which might even support that one theory, when Petunia makes reference to one boy who came around to the house. I mean, I had thought "Snape" when I first read the passage, although in all likelihood the most logical one for it to mean was James. Darn, fro not having the book and passage in front of me...
Harry and the Dark Lord. Neither one can fully live while the other survives. The prophecy could have led Voldemort to Neville Longbottom, as easily as Harry Potter. Yet, the Dark Lord marked Harry. That was important to the story. When that Avada was cast, and rebounded, it hit Voldemort. Some of the shock of that reverberation, that might have struck Harry's forehead, leaving that lightning bolt-shaped mark. A piece of Voldemort lodged itself in that scar. It might be that, when Voldemort is dead, the scar will disappear. Voldemort cannot live fully, because that piece of himself that is the scar's essence is a part of Harry Potter ,and apart from him. That part of the scar that is Harry Potter leads to the visions and sleepless nights, and leads to Harry knowing quite a bit--without fully realizing it--of what the Dark Lord is scheming. It interferes with Harry's life because the pains interrupt his sleep, and getting an account, though nor a full one, of what Voldemort is plotting--well, he's got a part of Voldemort's powers, but he also has powers of his own, and a mind of his own, independent of Voldemort.
Ah, anyway, that's a basic springboard to what I mean. I think this could apply to your own writing, as well. Even characters who never meet face-to-face might well affect one another through an object (The One Ring), or through a third person, or even an intermediary, without ever meeting face-to-face. These are the relationships that characterize your novel. You don't need a lot of that to start with (who could keep track?), but a short bit here and there might help your plotting, scheming, planning, and writing--and your characterizations.