Ruka is meant to be a foil to Chizuru and Kazuya, which is fine.
I would like her more if she had personal connection with Kazuya prior to meeting him.
She could have been written as an old middle school or high school schoolmate who was too shy to confess. She happens to get a second chance by running into him again, and from that point on everything makes sense, even the black mailing.
Anyways, the only thing that may save her character is we get some insight why she tries so hard for Kazuya. Maybe she made a promise to herself or something.
Other than that, I think the manga is pretty good.