Last one
Invisi Billy x Frankie Stein

A ship I also don't understand why people don't play around with? they're so complementary in colors...
I decided on this once I saw the episode with Rochelle and Invisi Billy just offers to help her even when he really doesn't need to (she has wings, everyone! her stone feet don't matter! she can fly!) and how that's Frankie's whole m.o., she's just out here to help her friends, so I got to thinking about how they'd be really sweet together, helpful couple

I do think of Frankie as poly, so I'm very open to shipping her with others, but right now it's just Invisi Billy

Frankie and Invisi Billy-Monster High