I only know vaguely of Sephiroth through osmosis (and the remake but who even knows where that's going) but maybe Emet will have cause to wander through those caves Sephiroth is in holed up in and run across him! So they could be occasional acquaintances.

Or else he just comes to know Sephiroth as that annoying ghost who shows up occasionally LMAO. Either way, I hope to see more of him!