This Arm Equipment raises Attack and Defense by 20. It is unlocked differently for male and female characters; for the former, it is unlocked after being bought for 1,700 Gold after earning 10 Honors/Achievements.
As for the latter, it can either be unlocked by purchasing it for 5,000 Gold after earning 20 Honors/Achievements, or it can be obtained on the 27th floor of the Tower of Lost Souls. To achieve this, the player must clear the stage without taking any damage.
This Arm Equipment is unlocked at the start of the game.