Context: this image is referencing the photo that leaked the SS86 Optimus figure before it's official unveil, which was a picture of the figure posed with a small piano. See below:
To those not in the know, Sango's/Manatsu's charm points are their cheeks/lips respectively when they transform, which explains why they're the one mimicking the Nyammy/Lillian transformation phrases.
This fanart looks directly based on the character visual from the Hell Girl live action (unless there's official art of Ai with the same exact pose and composition I'm not familiar with).