The narrator describes the mansion as a face with "vacant eye-like windows" and a "gloom" about it. What, other than the house, is he describing?
There does seem to be a psychic connection between the physical house that the Ushers live in, and the "house of Usher" as a family line. The house reflects, in its physical form, the fortunes of the family that built it. That family is now down to two people, Roderick and his sister Madeline. Madeline is dying and Roderick seems unlikely to have any children, so it appears the family line will die out with these two.
The "vacant eye-like windows" and the "gloom" of the house foreshadow and describe Roderick Usher. While his eyes are wild and filled with a strange light when the narrator first arrives, after Madeline's death and entombment, we learn that Roderick's eyes become like the windows of the house:
the luminousness of his eye had utterly gone out. ... I beheld him gazing upon vacancy for long hours ...
In other words, he is vacant eyed. Roderick, like the house, is also encased in a sense of gloom. As the narrator explains:
And thus, as a closer and still closer intimacy admitted me more unreservedly into the recesses of his spirit, the more bitterly did I perceive the futility of all attempt at cheering a mind from which darkness, as if an inherent positive quality, poured forth upon all objects of the moral and physical universe in one unceasing radiation of gloom.
It is as if the house and Roderick are one.
The most obvious answer could be the occupant of said house: Roderick Usher
Usher is a man plagued by many demons. His face is "cadaverousness of complexion; an eye large, liquid, and luminous beyond comparison" and his entire being is one of melancholy mixed with anxiety.
The less obvious answer but a good fit all the same, is Roderick's sister Madeline. She is prone to narcolepsy and falling into a death-like state. It is not much of a strech to believe that the Narrator could be desribing her vacant eyes and the gloomness about her being.
All of which sets up the ending, which is detailed in the link below. Hope this helps.