Rent-A-Girlfriend's Ruka Sarashina has added a welcome element of chaos and tension to the story. Not only did she find out that Chizuru is a rental girlfriend, but she blackmailed Kazuya with the information and insisted on becoming his actual girlfriend. This makes her come off as a little pushy, but she has her reasons.
Unfortunately, Season 1 of Rent-A-Girlfriend didn't really elaborate on what drives Ruka to such extremes. There are small hints sprinkled throughout the second half of the season, but none of them are fully explained. Season 2, however, may provide the opportunity to expand on Ruka and her motives. Here's what Season 1 implies about Ruka and what more there is to say about it in Season 2 and beyond.
One of the most notable habits of Ruka is checking her heartbeat. Whenever something potentially exciting happens, especially between her and Kazuya, she consults her heart monitor. Her goal is to reach at least 90bpm (beats per minute); when she's with Kazuya, that goal is reached and even surpassed. This ability of Kazuya to make Ruka's heart race is what attracts her to him.
When Ruka was confronted about her approach to Kazuya by Chizuru, she said it wasn't something she would understand. This is interesting as it implies a very personal reason for pursuing Kazuya, something she's not comfortable sharing. Once her reasons are revealed, they will likely add another layer of drama to the anime to keep audiences hooked.
Ruka's cause for pursuing Kazuya was actually heavily hinted at during the end credits sequence of episode 7, "Provisional Girlfriend and Girlfriend." There were shots of a young Ruka consulting a doctor, avoiding strenuous activities, riding a rollercoaster, being a rental girlfriend, checking her heart rate with each client, and meeting Kazuya. Some of these scenes have dialogue written down, but nothing too revealing.
As it turns out, most of the scenes from this sequence were taken straight from Chapter 28 of the manga, "The ‘Girlfriend’, Ruka Sarashina." As a child, Ruka was diagnosed with a medical condition that makes her heartbeat slower than average. She sought to elevate it with exciting activities, which eventually led to her looking for potential lovers among her rental girlfriend clients. She cried tears of joy over Kazuya because he was the only one to get her heart rate as high as he did. Most of this can be inferred from the Episode 7 ending, but it would still be nice to have the full scenes animated.
Chapter 28 has every right to be fully animated. As the anime suggests, the events of this chapter fall somewhere between episodes 7 and 8. Since this was only a one-cour season (~12 episodes), it's possible that this chapter was heavily trimmed to save on runtime. Season 2 could provide the necessary opportunity to elaborate on the events shown in the end credits sequence and give them their due attention.
Ruka's backstory definitely needs to be covered in more detail. It's crucial information for understanding her motives and desires. As nice as the end credits sequence is, it doesn't do the character justice. Properly explaining what compels her will add to the drama of the main story and garner her proper sympathy from the audience for her extreme measures. If Ruka is properly fleshed out by the anime, she could become even more of a fan favorite than she already is.
