Honestly, Ruri does not add anything positive to the story. She's there only to stall the development, because the writer is not good enough to do so convincingly. She is also necessary, because with how the two characters are (especially the fmc) they would've progressed far quicker.
So the author wants to have a cake and eat it too, a character that tries to progress (FMC) while also not allowing the story to progress, because it would require skill to then come up with something compelling for after that progress has happened.
And that is territory that is much less traveled, so you got fewer tropes you can shamelessly copy-paste...