Beep!

Soft-missable - D1G-g2r's questline runs through Xion and locks once you let Adam take you onward after returning from Abyss Levoire. Kick it off early and see it all the way through; his "CnL - Communicate and Link" step is also a prerequisite for Beyond Fate, so do it first. Miss the window and it's an NG+ job.