Walburga Black
"For a split second, Harry thought he was looking through a window, a window behind which an old woman in a black cap was screaming" (77) This is a woman trapped by her beliefs, hurt by them, and not able to do anything but rage. This is Walburga Black, of the "Most Ancient House of Black".
The glass in front of her painting is a perfect symbol for the notions that imprison her. Aristocracy can put a shine on any person, or if the window is stained, a dull color. It's up to the observer to decide if they are seeing the real picture or something distorted by glare or scratches. Lineage is as fragile as glass. One person lying about their background, a family dying out, less people around who fit the ideal, it's much easier to break a good lineage than it is to keep it. Pureblood pride keeps the outside world firmly distant from the real world, and that's a window that's much harder to open then a standard model.
When Sirius opens that window, Walburga flips out. She's spent her whole life entrenched in this culture of blood equaling status and power. She thinks that she has benefited from it, that her father's going back a thousand generations have benefitted from it. To see her firstborn son throw all that away to chase beliefs that she can't even comprehend hurts. Then her second son is killed for rebelling against those beliefs, ones he tried to hold for a little bit of time. Both sons throwing away beliefs that are generations old is hard enough, but to end up in a war and dead because of it? Maybe she thought that if they hadn't turned, they'd be safe. Maybe she wondered if she was weak, unfit, somehow flawed, for not being able to instil her sons with family pride. Maybe a thousand different things, but now both sons are gone and she's stuck with a war that pits all the old beliefs against all the new ones, and she's just lost both sons to the other side.
So it's a war, and she is somewhere between two sides of a war. Why didn't she choose one? The other side was completely unacceptable, no doubt. They wanted a new world, one where everything she was taught as a child didn't matter, or didn't exist. She was too entrenched in, and maybe just a bit too loyal to, her family's teachings to join them. But why not Voldemort? Maybe because one son was on the other side? Maybe because she saw her other son, barely more than a child, die because of misgivings? Maybe because her whole family was caught up in Voldemort's side, and she wanted to make sure someone would be there if the Death Eater's lost. Maybe she just didn't believe in the murder of muggleborns? It could have been any number or some combination of things. Not being able to pick a side, she couldn't defend her position, nor could she have the satisfaction of shattering her old notions into a hundred pieces. So she was stuck in the middle, and couldn't do anything but rant, rave and scream.
Which she does, to full effect. She's watching her house get used as a headquarters for these people who don't care a drop for family trees and age old bloodlines and the old families. They are intruders on her inner sanctum, throwing a stone through the window and letting themselves in. She's a dried up old painting, and the house is no longer hers, so all she can do is watch her house be used in a campaign against everything it stands for. Her entire life she's been told that she's got power because she's a Black, and she can't even protect her home. Or her sons. All that she can do is rage.
Citation:
Rowling, JK. Harry Potter and The Order of the Phoenix. Arthur A. Levine Books, 2003.
Used HP Lexicon to find the place.
Sorry that it's in so late and not quite what was asked for, but at least it's in, right?