Nice find, I bet this has existed since the first version without anyone discovering (or at least reporting) it. Seems like when you go to equip menu without a character in that slot, the game will generate a new one but it will have a zero grid size. And because they have no grid (and that isn't supposed to even happen) they are scaled way down and seem almost invisible.
Luckily that doesn't completely break the game. You just get a free flea sized party member who can't equip anything.