I just finished re-reading the novel Fifth Business again, and I can really see her in that role. In brief, it's an opera concept where you have the Hero, his opposite the Villain, the Heroine and her own opposite. Then you have a fifth character who might be on the hero's side or the villain's side, but they have the role of making the plot happen. They don't have a glamorous or impressive role, but they're still indispensable.
Maybe a bit heavy-sounding for something like a Yuru Yuri doujin, but it works for me.