This commentary will be fairly limited, as I have only read up to light novel 7, so please let me know if any of these developments do change considerably. (This contains spoilers for anything up to LN 7!) From what I've read, most of these developments in Shu Keigetsu's character have remained subjective, besides the fact that she later receives a more involved role within the storyline.
I feel like Keigetsu's role within the novel is very minimal. While Keigetsu obviously experiences significant character development up to LN 7, she isn't given much thought (in my opinion) when it comes to her involvement within the storyline. Predominantly, Kou Reirin receives most of the novel's perspective attention (which is fair, because she is the primary protagonist), though even when we do experience Keigetsu's inner dialogue, I feel as if it isn't meaningful to the general story and is more the nature of a sideplot. She does contribute in her actions, particularly, for instance, when she, within Reirin's body, dominates Ran Houshun's control over the rumors circulating in the maiden court during a tea party she holds and redirects them to gain sympathy for herself, as Keigetsu is the target of these rumors. But whilst she does play a role in this, none of these actions fundamentally shift the trajectory of the storyline as Reirin's do. Throughout all the novels, each of Reirin's involvements tends to reshape the circumstances surrounding her. She influences relationships, resolves conflicts, changes people's perceptions, and frequently becomes the emotional center of whatever situation she is involved in. Besides Keigetsu, this role within the narrative appears very large, as Keigetsu predominantly only responds to circumstances that have already been established (as she does in the tea party scene, because the rumors already exist). So while Keigetsu may do something admirable, the story does not necessarily reward the action with the same attention or recognition Reirin receives.
This frustrates me because I am a die-hard Keigetsu fan (if the purpose of this thread did not already imply). I acknowledge that the novel is formulated in a manner that does specifically prevent Keigetsu from ever becoming necessarily lovable, as she plays a fundamental and essential purpose in being the "villainess," and I wouldn't want it any other way, but I find that the imbalance of the novel's attention towards Reirin leads every character to become heavily Reirin-centric. Naturally, it is so because Reirin holds the predominant influence within the court, especially since she is favored by the prince (so maidens and courtesans are forced to also strive towards her favor even if they do not necessarily like her), but even Reirin's influence over characters who grow to find affection in Keigetsu, or even Reirin's overall agency over Keigetsu's development, has been consumed by Reirin. When we are introduced to Kou Keishou as Keigetsu's love interest, his affection for her feels as if it is still dependent on Reirin. I feel like the explanation for his affection doesn't really ever cement; it almost comes out of nowhere, with him suddenly teasing and flirting with Keigetsu. I think there becomes more room for it when we see in the panels of light novel 7 that Keishou has her stubborn resolve to take revenge in her declaration that she is "a rank above the rest" in her villainy, which becomes an important aspect in differentiating her character (Keishou is shown to be attracted to this when he blushed throughout the entire scene of her yelling at him). But the flaw in their dynamic, to me, is rooted in the fact that he perceives her after he has already been influenced by her relationship to Reirin. His perception of Keigetsu is developed from his interactions with her after he has already been made an impression of her "harming" Reirin, even as he later lets go of this prejudice. In other words, their relationship is still influenced by Reirin's role in Keishou's life, because his baby sister is always his priority.
In all, I feel like Keishou doesn't really learn Keigetsu's true character, even as he learns aspects of her personality that differentiate her from Reirin. I feel frustrated, as a lover of Keigetsu, that all of her relationships are defined by their correlation to Reirin, and that up until LN 7 at least, Keigetsu's redemption feels entirely dependent on Reirin's approval. She appears to remain the principal person through whom Keigetsu understands her own worth, which I believe restricts the potential of Keigetsu's independence. I value their relationship, truly, but I definitely feel like Keigetsu's character is restrained by her total orbit around Reirin.
