In Out of this world there are Sticker Packs in the shop that you can purchase using Soul Essence you acquire from beating nights.

When opening a Sticker Pack you get five sticker pieces. These sticker pieces are a 1/3 of a character's full sticker. By unlocking their full sticker you unlock them as a character. Certain characters are easier to get pieces for than others. Like VIRUS Wario may be easier to unlock than let's say VIRUS Entity01.

The same applies to Skin Packs that you can get from the other section in the shop.

The shop will unlock after you beat the first night of the Story Mode.