Fleda has no artifice about her: she is frank, honest, and acts with an unwavering sense of ethical commitment that is almost as single-minded -- though naturally more varied and nuanced -- as Mrs. Gereth's sense of artistic appreciation. She is a woman of ideas just as much as Mrs. Gereth is a woman consumed by her passion with things. In fact, the dichotomy that these two women represent can be seen in the opening chapter of the novel, when Mrs. Gereth "thingifies" Fleda by saying (or rather, with the narrator saying, though seeming to deliver Mrs. Gereth's inner thoughts), "Fleda Vetch was dressed with an idea, though perhaps not with much else" (Ch. I, par. 2). The near-nakedness of Fleda's ideas and ideals is seen time and time again throughout the Spoils of Poynton, as she attempts to manipulate Owen and Mrs. Gereth, by turns and one at the behest of the other, but does so with near total honesty to each. Even when she is not being entirely forthcoming her face reveals everything, especially to Mrs. Gereth, as Fleda's sensibilities run towards the internal world of ideas rather than the external world of things -- she is as incapable of betraying her inner sense of self and her ethical perspective as Mrs. Gereth is incapable of failing to maintain composure and control over herself. Fleda, to refer to Brown's distinction of terms, is more concerned with theory as opposed to either things or objects.
This is not to suggest that Fleda carries no traffic with the material world, hwoever, and in fact her friendship with Mrs. Gereth is built in no small part on the ingenue's appreciation for the elder woman's deft hand and eye when it comes to aesthetic presentation. Alan H. Roper describes Fleda's disgust on her first night at Ricks, the home to which Mrs. Gereth has retired so that Owen can occupy Poynton with his eventual wife, when she sees and feels it crammed full of the art collection that Mrs. Gereth refused to part with (p. 189). Left in Poynton, in the place it had been purposefully collected to fill and in which it had all been carefully placed and positioned, the effect had been awesome and had impressed Fleda; the same collection here was worse than tasteless but in refutation of taste. That it is emblematic of Mrs. Gereth's selfishness and lack of ethicality is no doubt a source of repugnance and doubt to Fleda, as well, but there is also a sheer aesthetic aversion to seeing works of art treated in this manner -- an aesthetic aversion that is tied to Fleda's ideas and not to Mrs. Gereth's concept of "things." Fleda wants a beautiful world just as much as Mrs. Gereth, and possibly even more so, and it pains her to an even greater degree than it does Mrs. Gereth to see the conscious ruining or degradation of that which in the proper context and with the proper freedom would be beautiful. It is an ethical wrong in the Aristotelian sense to see artworks treated thus, and it is through this idea of beauty and art that Fleda's sense of artistic appreciation comes about -- it is inseparable from her ethical perspective.
According to a reading of Henry James' notebooks, Fleda was initially intended to be a very minor character but took on a larger and larger role as the novel progressed, and especially as Owen fell in love with her (Baym, p. 102). She began as a thing of utility, a functionary in a plot that centered around Owen, his mother, and the potential for scandal at Poynton, but she emerged as an idealized and ideal-driven character who also ends up a study of isolation, after a fashion, due to her commitment to her ethical theories. Though she tries to manipulate both Owen and Mrs. Gereth she is largely unsuccessful with both, and there is little hope given of her ever being successful with either, she is quite honest in the way she goes about handling her manipulation. After confiding in Owen what she knows of his mother's plans, she continues with: "I shall instantly let your mother know," she again declared, "the way I've spoken of her to you" (Ch. VIII, par. 19). This is her frankness at work in a purely ethical capacity, and she has the same immediate and sure sense of right and wrong when it comes to her aesthetic sensibilities -- her revulsion at Ricks, her impressions of Poynton, and her impressions of Owen. It is within Fleda's grasp at several points in the narrative to acquire what she wants -- Owen and the titular spoils of the titular estate -- but to do so would require dishonesty or at the very least a disloyalty to her principles. This would be unbeautiful, ugly, and the bastardization of a good thing, and it is something Fleda cannot permit.
It is something Henry James could not permit, either, and thus the end of the Spoils of Pynton sees the house burning down with all of the artful treasures that had caused so much strife turned to ashes between its walls. Rather than have the home and the art turned over to the tasteless Owen and his new wife, the house -- apparently through mere happenstance -- is engulfed in flames, a far more fitting end to beauty than its degradation through unflattering display and tainted reputation. Mrs. Gereth does not suffer such an easy fate; she has had to age, to lose her beauty and her control over things of beauty, and over things at large. Fleda, too, is burned by these events to be sure, though like Poynton she has not been bastardized, and will not see herself degraded and deteriorated. Ideas, as Brown reminds us, are lonely in a way that things by their very thingness are not, but as Fleda demonstrates they can also endure.
Baym, Nina. Fleda Vetch and the Plot of the Spoils of Poynton. PMLA 84(1): 102-11.
Broderick, John C. Nature, Art, and Imagination in the Spoils of Poynton. Nineteenth Century Fiction 13(4): 295-312.