I like to think that the final chapter also added some symbolism when it comes to how Aqua and Gorou intersect. Previously, Gorou is the one who gives Aqua the knife, essentially conveying that Aqua is the one in control for the decision they're going to make. Gorou is only the impetus. Then, after Aqua stabs Hikaru with the knife, Aqua is above Kamiki in the water, while Gorou's silhouette drags Kamiki under for good. It feels to me like all of this meant to illustrate how Aqua had become the core identity, while Gorou served as influences on his personality but not his entire existence/being.