The trait tiers are Common, Uncommon, Rare, Enchanted, and Exclusive/Legendary.
Common traits are free. Regular MYO's come with access to all common traits.
Uncommon trait spells from the shop let you add an uncommon-level trait
Rare trait spells from the shop let you add a rare-level trait.
Enchanted traits include special features like glowing markings and auras. Traits spells for these are also available in the shop.
Exclusive/Legendary Traits are the most challenging to obtain, and are only accessible through special events or offers.
Affinity traits can be obtained by doing the Affinity prompts, or can be purchased in the shop.