Helms are classified based on their rarity: Common, Rare, Epic, and Legendary. The higher the rarity, the stronger the helm (they have better Shadow Ability and more perks can be attached to them). All helms, except for the common helms, come with Shadow ability.
Helms can be bought from the store. Otherwise, they can be obtained from chests won in duels, booster packs, or looted from defeated opponents. If the players receive a helm that already owned, a part of an upgrade bar will fill up. When the upgrade bar fully filled up, the helm will be upgraded, and a number of Shadow Energy will be given. Better chests and booster packs may contain helms with higher rarity. As for the drops from opponents, the players may obtain helms with random rarity; it is possible to get a helm with Epic or Legendary rarity, but the chance is small.
Special helms are limited-time helms that can only be bought from the store with a special currency. They are only available during special events update. Once obtained, special helms will stay with the players even if the event is already over. Their rarity are classified as Unique (comparable to Legendary) and comes with 2 slots.