The halfway mark of a series is often the place where two very different things can happen. Typically it's seen as the place to start in on the new story, the larger aspects of things by tying it all together or getting serious after some lighter moments and character introductions. Other times, more rarely these days compared to years before, it's the period where we get a recap episode that talks about what has happened to date in order to give everyone a mild break as well as providing an easy jump on point for new viewers that may be swayed by such things. Rainbow unfortunately goes for the recap episode as it focuses on Mario remembering everything that's happened which lead up to Sakuragi's death, with Sakuragi being the main focus throughout.
Which isn't a surprise. Even though this is largely an ensemble cast, Sakuragi dominated the series almost from the start with the way he befriended everyone by showing that he was the real power in cell block six. But even as he did that, he made them friends rather than have them cower in fear, he just wanted to make it clear where he stood and what he was capable of. Going back to those early episodes is a bit disheartening considering his fate, but it shows well how everyone came together over time and the bonds that were created are strong and powerful. Mario in particular, with him telling the tale, becomes a bit of a focus on things since it was him and Turtle that lasted the longest on the outside with Sakuragi, along with Setsuko, and that gives it all a bit more special meaning.