Complex ~Act.1 His Case~

I wondered the same thing but without a more explicit link I don't think we can say for sure - the girl in this story has bigger tits and we never find out the family name of Rumi from "Rumi-chan's Doing Her Best". It's entirely possible that Nagayori just likes the character design and name so much that they just reused them without really realizing. On the other hand it's possible that it's the same girl and she just got a little older and bustier, since her whole deal was basically being kind of dumb and letting men use her.

GAH...You might be right. If these titles are intertwined, I would have liked a story arc where that Sacchan chick was the manipulator in Rumi's orgies instead of involving an innocent bystander, the brother. It still would have been good but there's also an osananajimi-feeling element.