Sophia (the one in front of building that you originally offered to build shrine for) is the headmaiden. You need to have completed the beach event with Dea, Mai and Naevy before you can talk to her about the seal though, if you haven't done that.
As extra note, to get said beach event to trigger, must have done all of Naevy's quest lines and also events with Grey. Just adding that because I had same issue few days ago getting it to trigger, so worth noting.