Sakiko is the front and center of both of the promotional illustrations as well as the focus of both of the trailers we've gotten, so in a way I think that speaks for itself. The difference between Sakiko and Taki is that Sakiko is the one spinning the narrative, while Taki is living within Tomori's. Uika is also living in Sakiko's narrative and not the other way around. That said I wouldn't be shocked if the story was perhaps told from Uika's point of view and Sakiko was haunting her narrative so to speak, because this is what the stage plays in their concerts are like. Sakiko will always be the main character though; it's more comparable to RAS and how Chiyu takes priority over Rei when it comes to their narrative based stuff.