The Missing Partner is a quest in Xenoblade Chronicles. It can be received from Kurralth, who relocated to the Harict Chapel on Valak Mountain after the Mechonis Core event, but only if Adventurers in Peril was not completed. The The Missing Partner quest and the Adventurers in Peril Alcamoth Quest are indeed mutually exclusive. If Melia is in the active party, she will comment.
"Zain is safe. But what of the ruins he went up there to investigate...?"
- Upon completion of this quest, the Affinity Chart will change: