Tools of Characterization
Well, considering that the narrator spends the first third of the story talking about "my Ligeia," this one's a pretty easy call. Want to know where Ligeia was from? Boom, he tells you. Want to know what her voice was like? Bam, there you go. You don't even have to ask.
We hear a lot about how Ligeia looks… from the point of view of the narrator. He tells us about the color of her skin, the shape of her chin, the strange quality of her eyes; he goes so far as to describe her expression, the sort of I-don't-know-what that lies beyond the physical – or at least he tries to. The depth of these descriptions is highlighted when he tells us about Lady Rowena. Rowena, he tells us, is "fair-haired and blue-eyed" (18). That's really all the narrator thinks we need to know.
In the case of "Ligeia," we can narrow it down to a single habit, really: the narrator's addiction to opium. It may seem like a minor detail, but this addiction has major repercussions (don't they all?). For one thing, it lets us know just how badly he took the death of Ligeia. It seems as though he has to put himself into a drug-induced haze just to get through the day. Furthermore, because he is narrating the story, this little bit of knowledge can't help but make us wonder about the quality and truth of all the things he's telling us. In the end, his addiction doesn't just change our perception of him, it changes our perception of everything we're told.