What would happen if you didnt use a developer with a demi that "requires" it?

Absolutely nothing, the dye is oxidative so without peroxide, it would never activate, start working

It might stain a little bit initially if the dye has a lot of pigment (though most don't), but it would probably wash off on the next wash.

Girls are right, i tried it on my hair when it was white, i had bright pink demi that needs peroxide, i mixed it with conditioner to get paler pink and put it on my hair, it did turn out pale pale pink, but after a night of tossing around bed my hair was white again, i guess sheets took all the color out. So it wont stay even till the first wash, maybe if i didnt use conditioner it would, but it's way too thick to be used on its own.