He’s only introduced briefly and has like two lines, but I am really excited about including Sai into Broken Reflection. I’ve mentioned a few time before that I haven’t really seen all of Shippuuden, but in the parts I have seen I’ve really adored Sai. His personality, his history, and his growth were really interesting and I think he’s going to fit in well in BR.
For all of Natsuki’s doubts and fears, he was actually pretty good at the whole human thing. Yes, he tended to be overwhelmed by emotions and he didn’t always know how to deal with them, but he started to understand them and understood why people acted the way they did. And he cared. He cared a lot, which led to his rejection of himself (viewing himself as a monster due to his past actions/thoughts) and leading to his more self sacrificing decisions to protect the people he cared about from external enemies and eventually his more selfish decision of leaving to protect himself from those same people he cared about. (His own refusal to accept himself, of course, plays into this. After all, if he thought he was a monster then surely ever one else would think that too, right?)
Sai, on the other hand, despite being human, is not very good at the human thing. He needs help even if he doesn’t realize it at first. He’s been suppressing his emotions so long and has had so little interaction with other people that he does not know even the basics of human relationships. He doesn’t even know why he can’t quite understand why people do the things they do because of Danzo firm grip on him.
They are character foils in their struggles with humanity. Natsuki struggling with suppressing his overwhelming emotions till they reach a breaking point and ultimately rejecting humanity in a way by leaving with monsters (Oro and Kabuto) while Sai has to learn to reconnect with the emotions he was taught to suppress.
One of the themes of BR is that humanity has an incredible amount of emotion crammed into our tiny bodies. These two really expand upon that theme showing both the good (Sai learning to connect with his emotions by coming to care for others) and the bad (Natsuki being swallowed by his negative emotions as he walks down a treacherous path) side of humans having that emotional capacity. This shows humanities complicated dance with emotions and how people handle them in different ways.