Kirby is so innocent and cute that he can be included in yuri pictures without feeling like a third wheel.
Also he's small and round, so he doesn't take up much space.
We all know she's supposed to be a rabbit, but we call her a donkey anyway because she doesn't look like a rabbit. And donkey girls are more interesting.
I mean when a total stranger asks that to you and the cute girl who recently has started to come over to your house a lot for reasons that clearly have nothing to do with wanting to bang your mom, what are you supposed to say?