I like how Miho trusts Wanko enough to let down her fake nice personality around her, and Wanko has learned to not take Miho's salty remarks personally. It feels like they've become actual friends over the course of this manga.
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.