Sunday Mouri isn't actually there. He's just a figment of Sorin and Motonari's imaginations. Only Xavists and Ex-Xavists can see him.He's sort of like a ghost. Except that he's not actually dead, just the Xavist part of Mouri's heart that was rejected. That's why no one else has interacted with him; they can't even see him.
WMG / Desperate Setouchins