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The following are the rare heroes who, in my opinion, deserve not only emblems but whose limits are alright to be broken using aethers:
Original/Costumed Gunnar - Both versions are ideal for defense and offense, especially for the monthly challenge events (if it does not reflect ice). I only have one and plan to level another one (which will be emblemed in favor of defense and health on the original version) after stripping the first to reconfigure the talents in favor of the costumed version.
Nordri - He may be a poor elemental defense debuffer substitute (in case you don’t have Frida and/or King Arthur), but he is a viable alternative nonetheless. Like costumed Gunnar above, he will help improve scoring in the challenge events. I have 2 fully maxed and fully emblemed.
Original/Costumed Karil - this lone blue burn DOT dealer was greatly improved due to the costume. The original version have a better skill damage multiplier, but the costumed version has better attack stat. My least choice among the 3 blues.
Honorable Mention: Chick Jr. (for his massive AOE) and Frosty (for his healing and minions) or Helo (for his healing and cleanse).
Costumed Hawkmoon - The costumed version in Sorcerer class, if fully emblemed, charges her skills in 6 favorable tiles without even a mana troop supporting her during very fast wars and raid tourneys. I see no other viable options among the reds. Honorable mentions, for me, Vollermork (I know he is S4 but he is slow), Sudri (I know he is S3, but he is also slow), Bauchan (event hero) and Rudolph (event hero).
Grevle - Currently having the highest defense stat among the rare heroes, Grevle is valued not only for his overhealing but to his attack buff as well.
Original/Costumed Belith -
Original/Costumed Brienne - Who doesn’t want attack/defense buff?
Honorable mentions are Shrubbear (event hero), Hisan (event hero; I had at one point 5 of him maxed, now only 2) and Gramps (slow, similar to Alberich absent the heal over time).
Kvasir - A fast minion summoner ideal against enemy minion makers. I have 2 maxed and at +19 each.
Melia - Crit buffer, nuff said.
Poppy - Her possible additional hits up to 3 random enemies makes her the best sniper among the 3* yellows. I have 2 maxed and highly emblemed.
Honorable mentions: Costumed Gan Ju, Agnes, Costumed Kailani, Pixie and Arman. I don’t have the new yellow healer yet.
Bjorn - I like him. I have 2 of him maxed and highly emblemed.
Original/Costumed Tyrum - Dispel and cleanse are too rare among the rares.
Honorable mention: Gill-Ra, Original/Costumed Balthazar, Guardian Bat and Jack.