Gi

It hands you damage, critical strike chance, melee speed and movement speed at once, which no early chest piece does. The cost is the set bonus you give up by breaking up your armor. For melee and ranged builds that care more about killing things quickly than surviving a hit, wearing this between a helmet and leggings from another set is usually the stronger loadout right up until molten or necro.

The Traveling Merchant is the only source, so whether you get one is luck. Buy it the first time it shows up rather than waiting for a better day, because the stock rotates and he leaves.

It draws over the leg slot, so whatever you have on underneath is invisible while you wear it. If you were pairing it for the look, only the top half counts.

A gi is the training uniform worn in Japanese martial arts. The bonuses to speed and hit rate follow from that reading rather than from any armor logic.