Fena: Pirate Princess has only just ended yet has managed to leave many fans wishing for more, but not necessarily for good reasons. For a show that went out of its way to highlight the importance of its supporting cast at the beginning, it was entirely disappointing when that same cast barely had any impact in the end.
The three characters that drove the plot forward were Fena, Abel, and Yukimaru, even though the latter's only true relevance was to act as a romantic foil and bodyguard to Fena. Even Abel's role in the plot as its antagonist didn't do much to affect the story as for the majority of the show he was just following and chasing Fena. His whole character was there to tie the lore of the maiden that the show introduced towards the end to Fena and her journey, which is more than can be said for almost all the other characters.
Fena's samurai bodyguards were arguably the most affected by this lack of characterization and development. They may have received the most amount of screentime compared to the rest of the supporting cast, but that also makes their lack of depth all the more noticeable. While Yukimaru's origins and motivations are delved into, he's really the only one, with the others getting barely any attention in the story.
For example, the twins Kaede and Enju are referred to as "Ants" by Karin throughout the series, but there is never really any explanation as to why. Furthermore, why they were selected for this mission is also never elaborated on, which would help audiences understand why these two delinquents would be picked for such an important mission.
Karin is probably one of the most maligned characters in the story. As the only female member of the samurai in an anime that's supposed to feature strong female characters, she felt woefully ignored. Not only is she the only woman on the team aside from Fena, but she exclusively uses and prefers firearms over traditional samurai weaponry. This felt like it was absolutely ripe for further elaboration, as it could characterize Karin more but also explain why a samurai would use a gun. It could even have led to a deeper understanding of Karin's motivations and even hinted at how times were changing, even for her people.
Shitan is another prime example of wasted potential. While probably the most developed out of the crew aside from Yukimaru, he's depressingly one-dimensional. Shitan comes off as Yukimaru's fun and social counterpart, and it was very easy to like his character, but the problem is that it never really goes anywhere. There is a point where he does have an inner battle with what he perceives his true duties to be, the mission for his island or assisting Yukimaru, but it's resolved quite quickly and easily. Worse still, there are zero consequences for him choosing Yukimaru over his mission, even though the show kept warning the audience that there would be.
Unfortunately, the same treatment was given to Makaba and Tsubaki. Even though these two members often act as support for the rest of the team, they still deserved much more attention. Makaba is a craftsman who is actually the one who built Shitan's very hard-to-use bow. This could have been used to delve deeper into Makaba's character. The same can be said for Tsubaki, who is considered the weakest and worst fighter in the team. Why is he not as good? Does he have hidden potential? Are strategy and tactics his only skills? These are all questions that should have been answered and easily could have.
The antagonists get even worse treatment than the heroes in this regard. At the very start of the show, when the entire premise still isn't clear, viewers meet multiple villains that get very little characterization and even less explanation as to how they relate to the plot. Fena's body is bought in an auction by a man that is implied to be some kind of sexual deviant but later on, it appears that he and his "purchase" were somehow related to Abel. This gets little to no explanation and seemingly contradicts Abel's own actions and behavior later in the show.
There is also the pirate crew that first chases and even succeeds in capturing Fena during the first half of the show. Led by Captain Grace O'Malley, these pirates were built up to be quite interesting, particularly their captain. Considering she was working with Abel -- and sleeping with him to boot -- her role seemed to be quite significant. She even seemed rather emotionally betrayed by Abel when it became clear that he was only concerned with Fena and had no real attachment towards her. While definitely trope-ish, it developed her and even set her up to have a bigger role in the story, one where she wouldn't be used by men. But of course, she's assumedly killed almost immediately after this.
Fena: Pirate Princess is a beautiful show that unfortunately embodies the belief of "style over substance." While there are many interesting characters to be seen in the show, none of them ever get fleshed out or characterized in a way that allows them to fulfill their own potential. They become passive followers of a passive protagonist. Unfortunately, not even Fena's character was safe from this disservice, so there was never any reason to assume the side cast would be safe either.