Momo kills Suika when she comes back in order to protect Shamiko. Momo feels so guilty about not living up to Shamiko's expectations that she can never bring herself to become Shamiko's vassal. They stay together, though, and their relationship deepens. They go to college together, with Momo paying Shamiko's tuition, and get married after graduation.
Shamiko tries to change the Light Clan through activism after college, figuring if enough people learn to accept demons, the Light Clan will change to match people's perceptions of what is good and just. Momo stays by her side as her bodyguard.
The time finally comes, and the Light Clan does change—but not as hoped. Now it sees that humans have been corrupted, all demons are a threat that need to die, and the impure need to be purged. One-half of magical girls refuse to comply; the Light Clan terminates their contracts—Momo included—and the obedient half go on the attack.
Momo panics and goes into hiding with Shamiko, who's stopped aging because of her demonic nature. Shamiko and Momo live out their lives in exile, but Momo just keeps getting older while Shamiko doesn't. Momo finally dies of old age. Shamiko spends years alone depressed and grieving, training alone to strengthen herself, her empathy withering away in her grief.
By the time she's ready to face the world again, she finds civilization has collapsed from the Light Clan's oppressive inquisition—not just demons, but anyone who aids them are being killed. She wanders the world for a while and only finds abandoned ruined cities and walled fortress towns full of the last survivors.
She takes over a city with her powers, builds it into an empire, becomes a true Empress of Darkness, conquers the world, rounds up and genocides the world's magical girls, and seals away their Navigators. The world slowly rebuilds, peace reigns, and demons are safe, but Shamiko is left mourning Momo forever, lamenting she didn't want it to end this way.