Nana has shown very high respects and love for her sister, calling her "Ane-ue". They both excel in sports and get along quite well together, though sometimes Nana would get a little upset with her for her airheaded personality. Nana also shows respect for her when she scolds Momo for sleeping in Rito's bed, saying that Rito is Lala's fiancé, despite her annoyed feelings towards him.
Nana's most active and noticable relationship is with her twin sister Momo. While the two love each other and are often seem spending time together, they are naturally polar opposites. From an early age they have always been fighting and it was only through the efforts of her elder sister Lala that there relationship remained good. Although the twin sisters often squabble with each other over trivial things, they do love each other deeply. In To Love-Ru, they are often seen together and often plan mischievous things together, such as the virtual reality games, and running away from home to escape their studies together.
Nana envies Momo for having larger breasts than her, something that Momo doesn't hesitate to tease her about. Although Momo may have some reasons to be jealous of Nana, the only time she has ever shown jealousy towards Nana was when Rito hugged her, which is something Momo had never recived from him.
In To Love-Ru Darkness, they both enroll in the same school as Rito and Lala, and still do things together, but their lives become less attached and parallel to each other, and they are not as often seen together as they were before. While Nana develops her own friendship with Mea and lives a life far more relaxed than her younger half's, Momo concentrates on her Harem Plan that was originally proposed by Lala to make almost every girl close to fall for him, including her sister Nana. At the end of Darkness Chapter 35 Momo decides to let Nana in on her Harem plan, but when she hears of it, she is strongly against it.
Nana is one of the few girls who does not have romantic feelings for Rito in the beginning. In fact, her impression of him and his friendships with many girls is quite negative. She thinks of him as a shameless pervert, and is widely known for calling him a "beast". Overall, she doesn't genuinely hate him, but rather she can't seem to see what's so great about him. She has, however, said that he can be kind sometimes and is often grateful when he puts her needs before himself and tries to make her as calm and happy as he can, given her normally abrupt change of mood. She begins to have feelings for him when he saved her from falling.
After Nana's relationship with Mea became strained and left her heartbroken, Rito came to offer her advice and encouragement. When Rito hugged her in his arms to console her, it gave her the courage to confront and reconcile with Mea. Nana is grateful to him for encouraging her to confront to Mea, and she also notes that being hugged by Rito felt warm. Her feelings for Rito have greatly improved from this, and she subsequently snuck into Rito's room to sleep next to him (only to find Momo already sleeping next to him half-naked, much to her anger).
As time goes on for her, Nana eventually begins to have slight romantic feelings for Rito (like feeding him food), although she tends to snap herself back to what she thinks to be real and continues to dislike him. While she doesn't fully give into these thoughts, she tends to have a somewhat shy and flustered reaction to them.
Currently, Nana generally treats Rito like a brother, but obviously the kind she often squabbles with and gets mad at. She still says that Rito is an idiot, pervert, and a beast, but she acknowledges that he can truly be counted on when someone is in trouble and will thank him for his help, though in her own way and on very rare occasions. She remembers how good it felt when she was hugged in his arms. Once, while listening to a scary story, she easily got scared and was seen hiding closely behind Rito.
Despite what she thought of him the first time they met, she has slowly developed affectionate feelings towards him that she herself can barely understand or chooses to understand, much like Kotegawa. But later on, she wonders if she might be the type of girl Rito would like, and even wonders if he likes her breast size or if he would like her with makeup, however, when she and Momo where gonna see what kind of girls like, he tells her that he likes girls for their personality, not their appearences, and tells Nana that he doesn't care that she's flat-chested, which made her blush later on.
Interestingly, while she refers to him as "Rito", she referred to him as "Yuuki Rito" in her earlier appearances when talking to herself about him. This may show that she secretly didn't consider their relationship to be that close at the time. In later chapters of To Love-Ru and To Love-Ru Darkness, she always thinks and speaks of him as "Rito".
Sephie is the mother of Lala, Nana and Momo. It's said that she might be the most beautiful woman ever, something Lala and Momo have inherited from her, but not Nana as much (who is very sensitive about it). Momo states that she's a wonderful person. As the current queen of Deviluke (in their father's place, who isn't good at politics), she demonstrates active leadership and endeavors to maintain relations with every planet. To Nana and her sisters, she is the person they admire the most.
Nana has an obviously strong bond with her mother, hugging her in front of others and constantly being on the lookout for her well-being.
After enrolling into Sainan High School along with Momo, Mea became the first friend Nana made, and Nana greatly cherishes their friendship. Unknown to Nana, at first, Mea is a living weapon, like Yami (Golden Darkness). And like Yami, Mea is not genuinely accustomed to friendship and emotions, despite her cheery exterior.
Mea convinced herself that she is a weapon who was only pretending to have emotions to follow orders as a spy in the school. When Nana discovered Mea's identity as a living weapon, Mea confessed that she was only pretending to be friends. This left Nana deeply upset. But after consolation and encouragement from Rito, Nana approached Mea again, and Mea realized that she does still care for Nana. They have since become friends again.