Hyakkimaru’s prosthetics are used to show one of the themes in the show: duality.
Sometimes he’s a fluffy child, smelling flowers and rubbing his forehead on strangers to greet them. Sometimes he’s soaked in blood, slamming his head against a rock, and stabbing things out of rage. He’s not two different people though. He’s not being possessed or controlled. He’s just one, normally kind person, but, as the story progresses, he becomes increasingly unable to handle negative emotions and frustration.
Symbolically speaking, Hyakkimaru’s prosthetics are used to show this duality. The prosthetic arms, particularly when you can see the backs of the sword at his elbow draw attention to how he is different from others, but normally, when the blades are sheathed, he seems like the average person. It’s when the arms come off, revealing the swords, that he looks much more threatening to show his violent capabilities while he kills demons.
This also draws attention to scenes like this.
Hugging Mio with his sword arms shows how he never stops being human, never stops being himself, even when the symbolism tries to tell us the swords make him less so. His arms are fake. Whether he’s hugging Mio with sword arms or prosthetic arms, it feels the same to him.
Still, it’s no mistake that, as he is having an emotional meltdown while trying to save Dororo, one of his prosthetic arms breaks, exposing the sword. The worse shape his prosthetics are in, the worse he tends to be doing emotionally.
During his emotional breakdowns, he’s all swords. When something has gone wrong, something breaks.
Now in the final fight, both his arms are broken with only the sword parts left. Symbolically, he’s the worst off he can be. He can’t put his arms back on; his symbol of humanity is broken. I still see some hope though because because of the scene with Mio. He’s always felt, always been human, and he’s come out of rages before.
Hopefully, Dororo will be able to pull him back one more time.