Before I go into this, I believe the plot is pointing towards Chizuru and Kazuya together.
Having said that, I think, from a realistic POV, this is not good. Rental girlfriends provide a service. They sell the illusion of intimacy. While this isn't prostitution or other sex work, it does approach it.
Having Chizuru end up with Kazuya feeds a fantasy among a certain population that if you are just devoted enough and nice enough, that girl will like you. Even if you were paying for her attention. It may give hope to some that their favorite cam girl will fall for them, if they just try hard enough.
I think a far more realistic outcome would be Chizuru telling Kazuya that while she likes him as a friend, she is NOT attracted to him romantically. She's aware of his feelings for her, but she cannot reciprocate, and the stress she's been under is the fear of losing Kazuya as a friend.
The lesson here being, you aren't going to find a real relationship by paying for it. Money may improve your negotiating power, but it won't buy love.
So where does this lead our protagonist? Sure, Ruka is the obvious backup. She's awfully close to his perfect girlfriend. And you'd think that she'd do anything to swoop in an fill that void. But Kazuya has treated her like crap, and the realistic outcome would be for her to end the relationship between them. She found someone else (the director of the movie they worked on, for instance). Going the realistic route, she calls the relationship off after the trial, pulling the rug out from under Kazuya. He's devastated now.
Because you can't treat a girl that way for a year and expect her to wait around.
But Chizuru cares enough for Kazuya that she pays for a date with Sumi for him. Sumi learns what he's going through, and musters the courage to ask Kazuya if they could date in real life. Kazuya rebounds HARD with her, and finds out that he really likes her. REALLY likes her. They grow closer, Kazuya gets work in the movie industry, and they live happily ever after.
So Sumi wins the Kazuya sweepstakes, if this series wanted to teach it's readers a healthy lesson on how to treat women and develop relationships. But we all know that this is not going to happen.
