The following contains spoilers for Rent-A-Girlfriend, now streaming on Crunchyroll.
Rent-A-Girlfriend's Mami Nanami appears to behave like a sociopath and seemingly puts a lot of effort into ruining Kazuya Kinoshita's love life. She's known as the "Evil Ex" and extremely disliked by many fans of the series due to how toxic she appears to be. Though this hate can be justified by her actions, Mami's character is more complex than meets the eye, and it's highly plausible that she's just misunderstood by many viewers.
While actions speak louder than words, actions can also blind those who aren't paying attention to what's below the surface. Mami, like everyone, has her flaws, but she's been the most positive and pivotal drive forward when it comes to Kazuya's personal growth and maturity. She's undoubtedly the best thing that's ever happened to Kazuya.
Objectively Analyzing Mami Nanami's Actions and Behaviors
Objectively analyzing her actions thus far, Mami Nanami becomes Kazuya's girlfriend for a month and despite Kazuya's every effort to get closer to her physically, emotionally, and romantically, Mami pushed him away, but nobody really understands why.
Some would say that she's cold and is just using Kazuya. However, it is possible she avoids every physical interaction or romantic intimacy directed towards her because she's been traumatized in the past by someone else that had rushed or forced interactions with her.
She breaks up with Kazuya after a month saying she likes someone else. This is often a strategy people use to break up with each other when they feel uncomfortable in doing something. The reason she broke up with Kazuya after a month could be because of self-esteem issues or a traumatic event that happened in the past, and she just doesn't feel ready for a relationship. These feelings often don't show on the surface, and not many people would like to admit these things.
Additionally, in Episode 4 of Rent-A-Girlfriend when Kazuya is reflecting on his memories of Mami, she's told Kazuya that she had two relationships in high school which ended almost immediately. How these relationships ended could absolutely affect how Mami Nanami behaves.
She then blocks Kazuya on social media. This can be for many reasons, one of them being that she may not be able to handle seeing her ex-boyfriend on social media. She may feel that she would get hurt by watching from a distance, or she won't be able to move on.
Mami Nanami does some vile things in the series, such as trying to sabotage Kazuya's relationship with other girls. However, it is apparent through these actions that Mami Nanami still has feelings for Kazuya, and that she actually liked him to some degree despite what she says.
Many times throughout the series she shows her true face and emotions as bitter and cold, but this could also be a sign of trauma, depression, or loneliness. While Mami Nanami's actions and behaviors are brutal and downright mean to Kazuya, it's difficult to judge Mami Nanami without knowing her past.
Despite it all, Mami Nanami still shows genuine concern for Kazuya, and anger towards Chizuru for what she's doing to him. In Episode 12 of Rent-A-Girlfriend, Mami drops the truth bombs on Chizuru and tells her to drop the rental girlfriend business, and that she feels sorry for Kazuya because of the complications of him having a long-term rental girlfriend, he'll never have a real girlfriend.
She tells Chizuru that it's a loveless fake relationship with fake smiles, that exists merely because Kazuya is paying Chizuru to show her off as a girlfriend to others.
Why Mami Nanami Is The Best Thing That Happened To Kazuya Kinoshita
Every great character has had a catalyst to spearhead them to even greater heights and reach their full potential. Kazuya is no different. Rent-A-Girlfriend's story and Kazuya's love life started with Mami Nanami. Without Mami, Kazuya would most likely never have had a love life at all.
His friends are not doing very well finding love, and it is quite normal to be single throughout the college years. Rather than looking at what evil deeds Mami has committed, fans of the series can look at everything she's done to benefit Kazuya.
Kazuya and Mami shared many romantic experiences with each other, as Mami reminiscences in Episode 3. They've gone on multiple dates, even going so far as to pick out a name for their hypothetical child.
It's Mami who gave Kazuya his first real girlfriend and romantic love experience, first date, and first kiss. It's also Mami who gave Kazuya his first heartbreak. It's mainly because of Mami Nanami that Kazuya was motivated to build himself further.
Mami gave him the courage to meet other girls by making him so depressed that he would practice with other girls to get her back, even if he had to resort to using girlfriend experience services. This in turn caused incredible personal growth in Kazuya, and he was able to meet, gain experience, and charm Chizuru, Ruka, Sumi, and even Mami herself by the end of Season 1.
Mami Nanami was the perfect catalyst Kazuya needed to develop confidence, maturity, and initiative to develop himself and gain experience in the fields he was lacking in. As shown in Episode 12 of Rent-A-Girlfriend, even when they're not in a relationship together, Mami still shows some level of concern for Kazuya's well-being.
It could even be said that the strongest motivation for Mami's actions is because she genuinely cares about Kazuya. This is why Mami Nanami is undoubtedly the best girl in Rent-A-Girlfriend.