Will's Helmet

Developer outfits only come out of Hardmode treasure bags, which means an Expert or Master world and a boss you are willing to kill on repeat. Every Hardmode bag can roll this one except the one from Queen Slime, so pick whichever fight you can re-summon and finish fastest. When the roll finally lands you get the clothing and Will's Wings in the same bag, and the wings are functional gear rather than another social item.

Partial sets get you nothing beyond the look. With the Helmet, shirt and Leggings all worn, the character gives off a steady glow and trails a short after image behind every movement, so it reads best while you are running or flying rather than standing around in a base. Male and female sprites are identical, so there is no reason to weigh one against the other.

Will is Phlebas, an artist who worked on the game's graphics in earlier years. The set sat in the files for a long stretch with no way to obtain it, and back then wearing it was closer to a curse than a costume: any piece slowed you badly and kept a stack of debuffs pinned on you until you took it off. All of that was stripped out when the drop was turned on.