Gray has the habit of taking off his clothes(sometimes even without him noticing) because he was taught によって the Ice-Make Wizard named Ur, Gray, Ur and Lyon trained in the cold with their shirts off so that ended up being a habit.
Erza always wears her armour because she wants to シール her ハート, 心 to close her feelings also so she won't get tricked によって her feelings, she feels comfortable in her armour
Gray's and Leon's Master Ur taught him that "technique" and Leon only does the "technique" when Gray is around. Erza however is protect herself also because of Jellal and the Tower of Heaven
