Id say Andromeda or Sieg: Mordred will be gettin more and more redundant as we keep gettin more summons that can debuff and as we get more himes that can debuff. In the meantime Heals and resurrects will never get old, but as you already have Bryn and Amaterasu, who can keep your team healthy, Sieg can be a great choice, since boss cant whittle you down if its dead, and theres no better hero soul to make sure that boss is dead than Sieg.