Let me be upfront that, if she wasn't in High School, Ruka would be the best girl.
Having said that, Chizuru is clearly the romantic interest that MC will end up with. She likes him, but wont' admit it. He clearly likes her, but is afraid to admit it on the chance that she'll reject him (which is a stupid reason, but Kazuya is pretty stupid) and won't continue to go on rental dates with him.
The rental date issue is pretty awful. He's building up an emotional attachment here that isn't going to pay off under the terms of the contract. I think that the MC is written this way as a warning to other young men that this behavior is awful and won't lead anywhere. Ruka’s whole speech on that is pretty spot on.
In fact, to underscore this point, it would work better if Chizuru ended up rejecting Kazuya and he had to grow up, find someone new or commit to Ruka/Sami, gain a realization of what his problems were, and actually work on a relationship with the new main girl. That might actually be a learning moment for the simps in real life.
But who writes like that?