Canon: Sayori and Sephiroth will literally never see each other. End of story since nothing else matters
Fanon: Nothing here either
Peachy Headcanon: Sayori gets invited to Smash shortly before Min Min is announced. About 6 months later Sephiroth arrives uninvited and Sayori hates him at first. Over time, it becomes clearer and more apparent that Sephiroth genuinely cares about Sayori and perceives her as a daughter more than anything else. Over time, the [daughter-and-father-esque] bond between Sayori and Sephiroth becomes so mutual that it has started to pose a hefty threat to [almost] everyone around them since they started going to great lengths just to keep each other happy, and would no longer tolerate physical, mental, or emotional pain done to the other one. Any slight mistake would result in lethal consequences. Due to them having destructive intentions heroes from several different universes have joined forces to stop Sayori and Sephiroth
