WARNING: The following contains spoilers for Fena: Pirate Princess Episode 8, "A Knight's Vow," now streaming on Crunchyroll and Adult Swim.
It's no secret how much Shitan distrusts Fena. When he first meets her in Fena: Pirate Princess, he's kind and teaches her how to use the bow and arrow, despite how unwieldy the bow is and that it can only be used by Shitan. Their relationship is very brother/sister-like. However, after tasking Yukihisa to get rid of "the witch," Shitan turns decidedly less friendly toward her, especially when Fena's existence compromises Yukimaru's safety.
Although Shitan has relented and joined in the effort to save Fena and Yukimaru, this was clearly begrudging. If Yukimaru had not gone to save Fena, Shitan most likely would have dragged all of them back home and left Fena alone. However, Shitan now has to watch -- for the second time -- as Yukimaru is put in mortal danger because of this girl.
The first time was when he saw Yukimaru severely injured as the sole survivor of the Hope tragedy. Shitan had been extremely worried at the time, and Kei one day berates Shitan for being weak, calling him pathetic. Despondent, Shitan seeks out Makaba and examines the bows that are too stiff to be of use -- all rejects like him. Despite this, he still tries his hand at using these bows, although he obviously isn't able to hit the target. It isn't until Yukimaru makes an offhanded comment that Shitan should listen to his arrow and aim at the spot where the arrow wants to go that Shitan finally succeeds. It's the same advice that Shitan gave to Fena when they first met.
They manage to get Yukimaru to a doctor, who extracts the two bullets. Thanks to Fena's quick thinking of using tobacco leaves to prevent festering, Yukimaru will survive. While everyone celebrates, Shitan is the only one who stays quiet. He can't keep quiet for too long, though; he immediately blames everything on Fena -- Yukimaru's injuries, the fact that they're now all on the run -- and tells them that they have fallen under the "witch's spell."
Shitan tells them that before leaving, Yukihisa had informed him that Fena comes from a line of witches who leads men astray. Fena's own mother had driven a man mad, and it's safe to assume this man is Abel. Yukihisa has an inkling of Yukimaru's feelings toward Fena and warns Shitan that should Yukimaru fall under Fena's spell, he must kill her.
The reveal of Shitan's shared past with Yukimaru only highlights how important Yukimaru is to Shitan. It's because of him that Shitan is able to be the skilled archer he is today. His devotion to protecting Yukimaru stems from the ache that he felt at seeing him hurt and not being able to do anything about it. It's understandable, even if unwarranted, why Shitan does not trust Fena. He knows Yukimaru would do anything for Fena, especially if she were in danger, which consequentially puts him in danger as well.
It therefore makes little sense why Shitan has a sudden change of heart when Fena goes to see him. She argues that she can't feasibly be a witch and says that all she wants to do is travel the world with her friends. That's apparently enough for him, as he just pinches her cheeks and laughs, seemingly convinced.
The pacing in this episode feels off. While quieter episodes like these are needed to flesh out characters, half of it was split between Shitan's turning point and the heartfelt reunion between Fena and Yukimaru. The change in Shitan's attitude seems shallow given how strong his feelings are around Yukimaru; he was willing to kill Fena to protect Yukimaru. Nonetheless, Fena's earnestness and overall adorableness eventually win him over. He doesn't ever come to the realization himself that Fena is someone that he can trust -- he simply accepts that Yukimaru sees something in her, meaning his redemption arc feels oddly incomplete.