^^^^^ just made an account to say this. Sorry if my english is bad.
The short answer is that Ebrietas was the only Great One (divine and cosmic entities) that was left behind. After watching some Tsugu videos and seeing that she is lonely too, maybe the author tought it would be nice if these two had each other?
really nice shading, but proportions are kind of off. I'm betting that the heads are too big.
Also, if you applied enough pressure to pierce through armour your hand either would be in the gut because of impulse, or, if it was slow motion, awkwardly angled because given how high the tip of the sword red probably aimed from low point; she could have readjusted the grip but this would mean that at some point she just left sword in the blue lady, which is not very nice.
Pierced cape suggest swift motion, which means that she pulled out the hand out of her stomach, dragging sword with it. Yikes.
For what it's worth, while a lot of the Cinderella Girls anime is about the idols' relationship with their male producer, it's also about how they're good friends™. The anime for Idolmaster classic seems to have teased idol/P shipping, but Cinderella Girls does not. I've no complaints about lack of CG subtext, but I've been around for a long time so maybe my goggles are stronger than some.
(The Cinderella Theater shorts do hit producer romance, if that bothers you. That said, I got into the fandom via those in comic strip form, and they can get pretty subtexty when not directed at P/you.)
@juanelric You could go Natsuki with Ryo — they match pretty well as rock vocalists with some contrasting butch/femme energy. If you're too attached to the other age-gap ship of Ryo/Koume, consider Miyo as another biker-type. Or, since Natsuki has a kind of Western music/classic rock feel, you could ship her with Kate, the English girl, or Cathy, the American who can't speak English.
That said, Cinderellas do seem to pair off easily, don't they? It probably helps that the stable is so deep.