WARNING: The following contains spoilers for Rent-A-Girlfriend Season 1, now streaming on Crunchyroll.
After series protagonist Kazuya Kinoshita and his rental girlfriend Chizuru Mizuhara get mixed up in a string of misunderstandings with Kazuya's family and friends, they continue posing as a real couple in order to protect important secrets. But Ruka Sarashina, a fellow rental girlfriend who was rented by one of Kazuya's friends, discovers their secret and uses it to blackmail Kazuya into a legitimate relationship.
Ruka's behavior at times seems bizarre, even by the standards of Rent-A-Girlfriend's colorful cast. Despite labeling Kazuya a pervert shortly after their first meeting, she grows absolutely distraught when he initially turns down her relationship request. She also measures her romantic feelings via a heart rate monitor. This all appears nonsensical on the surface, but there are clues in one episode that could explain Ruka's motivations while hinting at a darker backstory and future.
The end credits are normally as silly and lighthearted as the show itself often is, but the Episode 7 outro changes that by showing a younger Ruka in the hospital with her doctor looking upset. The next scene shows an injured Ruka lying on the ground in gym class. A third scene shows an adult woman, presumably Ruka's mother, taking away her game console and saying Ruka can't get overexcited. As the credits move chronologically into her teenage years, she listens to other girls talk about major life events like kissing a boy and how fast their hearts were racing.
When put together, the scenes suggest Ruka suffers from a serious heart condition and cannot do anything that risks making her heart race too quickly. She's shown alone and crying multiple times, so it's not surprising that a loving relationship is something she desperately wants. It also explains why she works as a rental girlfriend, being able to meet different types of guys and find one who gives her that same heart-racing feeling her classmates were swooning over. And if her heart rate isn't supposed to go past a certain number, that would explain why she measures her potential romantic feelings via a BPM (beats per minute) monitor. Her "goal" is 90 BPM, which Kazuya clears when Ruka puts his arm around her.
Although Ruka gets the provisional relationship she wants with Kazuya, he often treats her like a pest who simply has to be dealt with in order to protect Chizuru. Ruka, however, is usually jubilant when she's with Kazuya and even takes him to a love hotel before deciding she's moving too fast. But the fact that she's constantly so gung-ho, even after achieving her relationship goal, could be a hint that Ruka's condition is very serious and she wants to live her life to the fullest while she still can.
As lighthearted, funny and absurd as Rent-A-Girlfriend can be, it's also a roller coaster ride with surprising twists and turns. If it turns out that Ruka's situation is as serious as the one-off end credits suggest, it would explain much of her behavior so far while adding a sobering new layer to her story. With Ruka getting ever more serious about a lasting relationship with Kazuya, he will surely learn about her past as she opens up to him. Chizuru is the one Kazuya loves, but how might his feelings and judgment be affected if he learns the girl fighting hardest for him has a serious heart condition?