Kazuya's family's sheer excitement to see him have Chizuru as a girlfriend. Even if part of is due to them seeing him as a loser, and it ultimately results in Kazuya having to spin a web of lies to avoid letting them down, it's still sweet to see them be happy about their son falling in love, as opposed to viewing the girlfriend with suspicion. For all of Kazuya's flaws, he can't bear to break his family's hearts.
In Episode 4, Kibe punches Kazuya for fawning over Mami when Chizuru is apparently his girlfriend, then later goes to talk to Chizuru about Kazuya. He doesn't know the entire story, but he's not wrong that Chizuru's a better girlfriend for Kazuya, and shows how much he cares for Kazuya.
In Episode 8, Chizuru gives a well-chosen gift for Kazuya- a phone case, since Kazuya's phone has a cracked screen and he doesn't have a case. Considering that the phone got the crack when Kazuya dropped it before diving to save Chizuru, it's a small but effective gesture, even if Chizuru refuses to call it a gift.
In Episode 9, Ruka listens to Nagomi ask about Chizuru, since she mistakenly believes that Ruka and Chizuru are friends. Nagomi admits that she's put Chizuru on a pedestal, but she wants to know more about her, and later shows concern when Chizuru catches up to her out of breath. Ruka realizes how much the Kinoshita family cares for Chizuru, which presumably influences her decision not to tell them about Chizuru's rental girlfriend job.
When Kazuya notices that Kuri is bitter about Ruka outing herself as Kuri's rental girlfriend in front of Kazuya, he takes action. Kazuya pays to rent Chizuru for Kuri, despite the fact that would mean admitting Chizuru is also a rental girlfriend, and he's broke enough to need a part-time job. His plan works, and Kuri, understanding the significance of the gesture, feels better. They also laugh it up about renting a girlfriend, and Kuri decides to keep Kazuya renting Chizuru a secret.
Chizuru and Sumi's friendship. Chizuru wants to help Sumi get better at her job, so she asks Kazuya to take Sumi on a date so Sumi can get some practice with a client Chizuru knows. Sumi, despite having feelings for Kazuya, doesn't try to come between him and Chizuru, but instead wants to help them however she can.
Even though he finds her difficult to work with at first, Kazuya does whatever he can to help Sumi come out of her shell and feel more comfortable with herself. At the end of the date, he tells her that she's not a bad person, echoing Chizuru's sentiments that there are people who will like her for who she is.
After Mami finds out that Chizuru is a rental girlfriend and rents her for a date, Chizuru once again stands up for Kazuya and makes one last and earnest attempt to convince Mami that he could perhaps be the one person who could make her happy for the rest of her life. Although she doesn't show it on the surface, deep down Chizuru does look out for him and stands up for him, while acknowledging that he's better than he thinks he is.
When Chizuru risks getting exposed at her birthday party because of the drinking games everyone else is playing, Kazuya loses on purpose and ends up getting extremely drunk so she doesn't. Realizing he struck out on purpose to protect her secret, she later finds him, helps him back into his apartment, and comforts him while he vomits into his toilet. She even tucks him into bed before she leaves.
Sumi's special date for Kazuya. She allows him to revel in his love for marine wildlife by taking him to the aquarium, a dolphin show, and going fishing. When he realizes that Sumi went above and beyond in planning out the whole date, he finds out she did it to celebrate his birthday. Thinking Sumi had grown and he hadn't, she lends him an ear and listens to how badly he wants to help Chizuru, and in their tears, she assures him that she's also there for him. As they depart, he tells her that it was the best date ever, making her happy that her efforts paid off.
During the filming project, Kazuya keeps his distance from Chizuru so he doesn't accidentally interfere with her work, which she misinterprets as him being upset with her. When she confronts him about it, he clears up the misunderstanding while mentioning that watching her in her element makes him feel inadequate. She not only tells him that being an actress doesn't make her any better than him being a future shop manager, but also tells him that after everything that's happened between them, she's never regretted meeting him. Not even once.
The various times Chizuru comes out of her shell around Kazuya and shows her kinder and more playful side to him, even if it's a small gesture like buying him a bento on the train, getting him a soda after the film premiere, and sneaking up on him from behind just for fun. Even if Kazuya has no idea what he is to her, which isn't helped by how guarded she is around him, it's nice to see moments where she treats him like a good friend and it's more obvious than usual that he's much more than just another client to her.
Kazuya finally getting Chizuru to lower her barrier and let out her tears by describing her as his ideal girlfriend, while assuring her that it was okay to cry. She acknowledges the next day that it actually helped her.
After Chizuru realizes that Kazuya was actually describing her when talking about his "Perfect Girlfriend", she starts making a more conscious attempt to close the gap between them. Even after his confession gets interrupted by Kibe, she invites him to Umi's party, apologizes when her friends drag her off, and comes back with food so they can eat together. She even reassures him that she was the one who invited him and won't let him be alone by himself.
The "Children's Day" one-shot depicts the main characters as kindergarteners but with all their same quirks. At the end, Chizuru still collects money from Kazuya, leaving him to think he doesn't mean anything to her, but she's quick to put it into her secret box that she doesn't tell anybody about. The last panel reveals what it's for: "Chizuru and Kazuya-kun's wedding fund."
At the spa resort, Nagomi finally has a chance to talk to Chizuru about Sayuri's passing, and asks if Kazuya was able to help her out in any way despite how helpless he can be. Chizuru tells her that he helped her a great deal, and Nagomi tells her that their family all want her to be happy and that they're there for her whenever she's down. Nagomi then tells her that she doesn't want her to think she's alone without a family because she has one in them. Chizuru's expression really tells touched she is, as well as how alone and afraid she's felt since her grandmother passed.
In the first chapter of Rent-A-(Really Shy!)-Girlfriend, one can't help but feel as happy as Sumi is once she finally gets her precious French Angel donut.
In Chapter 6, when Sumi's feeling depressed over getting a one-star review from her latest client, Chizuru calls and tells him about how she's found someone who can help Sumi with her communication issues (i.e. Kazuya). Since the rest of the chapter shows how much Sumi is struggling with her job, it's all the more heartwarming that Chizuru was able to get Kazuya to help Sumi.