WARNING: The following contains spoilers for Season 1 of Rent-A-Girlfriend, now streaming on Crunchyroll.
Kazuya Kinoshita, the utterly hapless protagonist of Rent-A-Girlfriend, starts the series by getting dumped by his girlfriend. Feeling lonely and spiteful, he decides to try the rental girlfriend business and meets Chizuru Mizuhara. Subsequent misunderstandings with Kazuya's family and friends lead Chizuru to continue posing as his real girlfriend to protect both of them. Their misadventures grow wackier and more dramatic as the lies pile up and romantic feelings begin to bloom.
Kazuya's ex, Mami Nanami, discovers that Chizuru is really a rental girlfriend and blackmails Chizuru for her own twisted pleasure. Mami is in many ways the antagonist of the series so far, but she makes an important point to Chizuru during their showdown talk: by lying and posing as his rental girlfriend for so long, she's really been hurting Kazuya more than helping him. And it turns out Mami is actually right.
Kazuya initially rents a girlfriend because he wants to stop feeling lonely. The desire for companionship is a common reason for renting a girlfriend, along with potentially learning more about dating, how to act, what not to do, etc. But Kazuya's decision is purely out of vanity, with no real thought or substance behind it.
When a misunderstanding has Kazuya's family and Chizuru's grandmother believes that she is his legitimate girlfriend, Kazuya can't bear to tell them the truth, so he claims Chizuru is his girlfriend -- and she goes along with it. Throughout the season she chastises him and insists they need to come clean and go their separate ways but continues going along with the deception, thus enabling his behavior.
Therein lies the moral debate: how far is a rented girlfriend supposed to go in aiding her client's deceit, especially when it negatively affects her own personal life? On a trip to the beach, Chizuru even lies to her own friends by pretending to meet Kazuya for the first time. Where is the line drawn between making money (and gaining valuable experience as she pursues an acting career) vs. refusing to aid in lies that don't help her client in any way? They spend much of the season desperately scrambling to keep their secrets while Kazuya frustrates with his inability to learn from their (mis)adventures and mishaps.
This is all put into stark focus with the arrival of Ruka Sarashina, a rental girlfriend who genuinely falls in love with Kazuya and blackmails him into a relationship. Ruka quits her job to show her devotion and tells Kazuya their relationship can be provisional. All she really wants is a chance. Annoyed by Kazuya not taking her seriously, Ruka asks Chizuru why she's still around since Kazuya now has a legitimate (if provisional) girlfriend.
Chizuru, of course, has shown hints of starting to like Kazuya and gives a non-committal answer. Even if Ruka isn't the girlfriend Kazuya wants, the subsequent relationship was a prime opportunity to gain some valuable life and dating experience -- the kinds of lessons he hasn't gotten from a rental girlfriend relationship. But because Chizuru, his real crush, continues to be his rental girlfriend, Ruka is often treated as a pest instead.
This isn't to lay all the blame for Kazuya's lack of growth on Chizuru, of course. Ultimately he alone must take responsibility for his actions and mistakes. And if he wants to be worthy of a proper relationship with Chizuru or any girl someday, he has to grow up first. His lack of development is magnified in the Season 1 finale when he nearly accepts a permanent relationship with Ruka before trying and failing to confess to Chizuru.
On the flip side, Chizuru's contradictory chastising and then enabling his poor choices shows that she also has room for growth. She could refuse to go on any more rental dates with him and tell her own grandmother they broke up, thus ending her stress while keeping her rental girlfriend job a secret. As Kazuya notes, she's an extremely popular rental girlfriend and has no shortage of clients. Instead, as Kazuya increasingly relies on her help to continue lying, she becomes his crutch. And as he develops genuine feelings for her, he finds it even harder to let her go.
In the finale, Chizuru lands a major acting role and questions whether it's time to quit her rental girlfriend job. This would do them both good, allowing Chizuru to extricate herself and her identity from Kazuya to pursue her dream. Meanwhile, her absence from Kazuya's life may force him to finally come clean on all the lying and start fresh. The story is built around Kazuya's growing feelings for Chizuru and hinting that they could be reciprocated. But before they can start to grow together, they need to grow individually first.
