So basically in this world, in which you can see all character you want...They're fused. It's always a fusion of two people (unless you're talking about Steven Universe i guess? Some gems can fuse as 3 or 4 people i guess). Either through the Fusion Dance, or the Potara Earings. However the catch is that, the fusion is permanent, which explains why only a very few people (or even only you?) aren't fused with someone. Have fun in this chaotic world!