She's also Momochi's mole mate.
Come on, someone else had to notice it too.
Actually, I have a pet theory that posits that Miwa is Tenmangu in a modified android body (possibly. This theory assumes that something traumatic happened to Old-Miwa's body and/or consciousness, like most other theories out there) while Momochi is Original Miwa, erased of her memories (purposefully or by said trauma). Even without the lost-memories trope, she could be serving as a double agent for Kabapu, enabling Now-Miwa plays both sides of the (hopefully) upcoming epic Il Pala-Kabapu battle for the ruins.
Hmm. Unlikely, yes. But it would explain some of her reticence to have contact with Shioji, because of a potent mixture of latent guilt (Poor Shioji got stuck with Miwa as a mommy), general weirdness, and nostalgia.