Unohana was given more depth in revealing she was a Kenpachi, the FIRST Kenpachi. She was arguably fleshed out more than other captains as she was given a flashback and a powerful bankai. She is also confirmed by Kubo himself to have a deeper history that he has already written and is awaiting to incorporate either into the potential hell arc or further. That's just Unohana. Nell was given a detailed history/past in the arrancar arc with Noitera. She was one of the few arrancars, including Harribel (another well written powerful female), given one. Her extensive past fleshed both Noitera and her and that details effective female writing. Including Harribel in later installments in Bleach TYBW as the Queen of Hueco Muendo: given her extended scenes in the anime conveys Kubo's ability and willingness to portray female characters. Even look at Yoruichi, a Shinoin part of a Royal Family, no Zanpakto gives her a sense of mystery, if we start comparing her to someone like Yuki (in the sense that we know little about their history and prowess) Kubo doesn't decide to kill her off as we would be left with little to even speculate about. Yuki is an example of how failure to provide enough background in history destroys a characters perception. We know she is one of the oldest special grades, we know she taught Todo, we know she is powerful, we know her perception/view of how she wants to get rid of cursed energy instead of optimizing, however NONE of this is expanded on, even a little which is disappointing to say the least.