Ghostar's Soul Jar

Developer outfits only come out of treasure bags, and treasure bags only exist in Expert and Master worlds. A Classic playthrough cannot produce this set at all, however many bosses you take down.

Any Hardmode boss bag can roll it apart from Queen Slime's, so the quickest route is to repeat whichever Hardmode boss you can already kill in a couple of minutes rather than holding out for the late fights. The chance is low enough that it is something you pick up while farming bags for other reasons.

A successful roll hands over the whole outfit at once, soul jar, garb and tights together. Ghostar's Infinity Eight drops alongside the clothes rather than in place of them, and that one is not vanity, so the roll is worth something even if the outfit never leaves your storage.

The clothes sit in the social slots and change nothing. The set is the in-game likeness of Ghostar, who worked at Re-Logic as a production assistant, one of a group of developer outfits that each stand in for a real person on the team.