If it’s not Regina (who seems the most logical choice given the direction of the season) then I think it’s Charming since 1) his episode and 2) he’s following in Arthur’s footsteps as a heroic knight AND is the guy who will be getting “the relic” in the past (I assume). Meaning that the Siege Perilous will be offered to him, making him a knight of the Round Table and worthy to draw forth Excalibur.
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"He was a lot of things to me" "The only conclusion was love"