This video is by SchattenKitsune666, and features a duet between Fox and Krystal. Very sweet! The song is "All We Know" by The Chainsmokers - BILLbilly01 ft. Alyn and Violette Wautier Cover.
Turns out there's at least one other Krystal MMD rig. I'll have info about the other MMD rig later.