Robot Mask

Halloween is the only source. While the event runs in your world, any enemy can drop a Goodie Bag, and a bag can roll one of the robot parts. All three roll independently, so finishing the outfit is a question of volume rather than luck on a single bag.

An ordinary spawn farm with a Battle Potion going does this faster than chasing any particular enemy.

There is a quirk in the walk animation. The shirt holds your arms straight out in front of you as you move, so the character stomps around like a wind up toy. Some people wear that piece on its own purely for the effect.

Male and female characters are drawn identically in it, and the whole set is social slot only, so nothing about it changes how you fight.