Yami was initially hired to kill Rito by Lacospo (one of Lala's suitors), but after learning that Rito is not Lacospo described him to be, she turned on him, but still claims that she will kill Rito eventually and will stay on earth until her mission complete (even though the job is technically over). Later, she actually confirms to Rito that she only said that as an excuse to stay on Earth, and warns him not to make her too angry (Chapter 138). As time passes, Yami seems to grow some feelings for Rito. It is Rito who introduced her to taiyaki, the only thing she really eats. Rito carried her on his back to Mikado's clinic when she overused her powers, saved her from another assassin hired by Lacospo, and, unbeknownst to him, showed her a kinder side of himself that made her envy Mikan for being so close to him. Yami hates it when Rito sees her naked or changing, and she never hesitates to use her transformation powers to give him a powerful punch to the face, though this is possibly just her instinctive way of getting someone to leave her alone. She does not seem to mind when other girls see her naked or changing.
Yami appears to have love-hate feelings towards Rito. She convinces him, and perhaps herself as well, that he is still her target. And she often states how much she hates perverts. Naturally, Rito's pattern of accidentally falling and groping girls earns Yami's wrath more than a few times. Even when seeing Yami naked is clearly not Rito's fault, she would still proceed to give him a beating. However, like other female characters, Rito's kindness does manage to slowly grow on her. It's possible that Yami knows that Rito isn't as perverted as some think and just hates it when he sees her without clothes.In Chapter 158 of the manga, Yami was accidentally sprayed by Celine's pollens, causing her to feel infatuated with Rito. She then forces Rito to spend time with her (like a date), and soon after, asked him to kiss her, but stopped her and tried to get her to go back to her old, normal personality. After the effects of the pollen wore off, Yami asked why he didn't kiss her. Rito replied that he couldn't do that if she was not on her proper mental state. Yami then called him idiot for showing kindness to an enemy, but also made it clear that she doesn't hate idiots. She then smiled very sweetly at him. This is the only time Yami ever showed her soft side to a male, and also the only time she smiled heartily in the whole manga. All of this suggests that it seems possible that Yami started to develop feelings for Rito from this point. Even though she says she'll let him die painlessly when she kills him, obviously as a cover.
In To Love-Ru Darkness (the sequel of the original To Love-Ru manga), while Yami is still wrathful to Rito's usual clumsiness, her relationship and feelings with him continue to grow and develop.
In Chapters 013-014, Rito and Yami get caught by Lala's invention and become stuck together, temporarily unable to seperate, much to her bitterness. After a day putting up with each other, when things finally calm down at night, they are finally able to speak more calmly with each other while being alone together. Yami begins revealing to Rito about her past with Tearju, and the organization that separated them and turned Yami into a weapon, something she never spoke about even to Mikan. When Rito condemns the organization, she finds him quite strange because even though Rito knows that he is her target to kill, he would get mad for her sake at the people from the organization who used her like a weapon. Yami notes that she feels safe and at peace when she is around Rito, feelings she does not often experience in her unhappy life as an assassin. After the effects of Lala's invention wore off, Yami still clings to Rito as she falls asleep. Rito is the only person whom she has opened up to about her past.
In Darkness chapter 40, Yami's Darkness transformation radically changes her personality into a bolder and highly perverted one, and her feelings for Rito are revealed (or somewhat changed) in a very psychotic way. She now greatly enjoys feeling ecchi, and immediately teleports Rito to her side, surrounding him with many girls as she molests them around Rito. She explains her desire to kill him in a very ecchi situation to express her love for him and because he is her target.
It is revealed in Chapter 46 that Yami is in fact in love with Rito, and that it is said love that causes the "bug" in her Darkness transformation. But after Rito helped her transform back to normal, she denies any kind of love for him and still insists that he is her target. Nevertheless, Yami is grateful to him and now possesses a desire to improve her relationship with him, though due to her growing tsundere traits, she is far from willing to admit it, and Rito ignorantly interprets her angry outbursts as still hating and wanting to kill him.
After that incident, Yami realizes that her feeling for Rito is love. She admits to Mikan that she is in love with her older brother. In Darkness chapter 71, Yami transforms into her darkness form to save Rito from Kuro. After the fight, in chapter 72, Yami confesses to Rito by calling him her "love target". During this incident, Yami is groped by Rito but this time she just scolds him a little bit, instead of hitting him like she's been doing in the past.
Yami's most significant friendship is with Rito's younger sister, Mikan Yuuki. This may be because of Mikan's frequent friendly advances on Yami, and Mikan constantly talking with and helping Yami. In Chapter 139, she and Mikan use Lala's invention to switch bodies for a day, both wanting to see what the other's life was like, and Yami envies the life and relationship Mikan has with Rito, and considers the brief experience as a precious memory. In Rito and Mikan, she sees the peaceful life she could have had, and desires deep down. Mikan is the only person Yami truly considers her friend and is seen by her side often.
Yami is forced to fight Lala when she is assigned to kill Rito. After they stop fighting, they become friends (at least in Lala's eyes). As of Yami's opinion of Lala, she seems oblivious to Lala's friendly behavior towards her, but she never attacks Lala or says anything bad about her after their fight.
It was Lala who named her Yami-chan (Yami means darkness in Japanese), a name repeated by most other characters.
In To Love-Ru Darkness, Yami's Darkness transformation activates, radically changing her personality. With her newfound shameless personality, she psychotically expresses her love for Rito and her desire to kill him, and with Lala protecting Rito, Dakrness declares Lala as her enemy and the one blocking her from killing Rito and become one with him. But after Yami's transformation is faded, she apologizes to Lala for their fight, which Lala quickly forgives without any problem.
Mea still thinks that Yami should be the ultimate weapon and unstoppable by anyone. However, in To-Love-Ru Darkness chapter 30, Mea has a slight change of heart after her conversation with Rito. When he explains how Tearju sees her and Yami as her little sisters and how families are important, she helps him persuade Yami to come back to school and perhaps rebuild her relationship with Tearju.
In Darkness Chapter 37, Mea was forced to use her transformation powers in front of many others to save the school from a micro black hole. Yami then appeared and introduced Mea as her little sister. Mea was then warmly welcomed by all of the students present. Yami has shown that Mea is someone important to her , some examples were when Nemesis lied to her about Mea not being real which caused her to lose her peace of mind and use her "Darkness" ability. Another is when she had transported Momo and Nana to Mea's apartment while she was "Darkness" showing she still believes Mea is real.
Tearju is the one who created and gave life to Yami from her own DNA, making Tearju sort of like her mother, but Yami stated they were more like sisters. Before her life as an assassin, Yami used to play and have fun with Tearju and would even eat whatever she cooked (despite being badly made). Yami seems to hold high respect for Tearju even now. When Tearju escaped the organization, she was unable to take Yami with her, although Yami doesn't blame her for it and says she's glad that Tearju is alright. Tearju wants to be close to her again, but because both are similarly emotionally awkward and unsure, neither would approach each other unless forced to. Since Tearju's arrival as a teacher at Sainan high, Yami stopped wearing her uniform and stopped attending classes.
If Tearju had rescued Yami from the organization that turned her into a weapon, Yami would have grown up differently with a better and happier childhood. It is revealed that Yami is afraid of becoming closer to Tearju because she doesn't know how to face her after becoming a notorious assassin, and feels she doesn't deserve to be with her again after what she has become. But with help of Rito and Mea, they persuade her to come back to school. Tearju has become her new homeroom teacher, and was tearfully overjoyed to see her in her uniform.
Yami's relationship with Tearju still remains very awkward. Even after Yami publicly introduced Mea as her sister, she remains reluctant to become closer to Tearju again and would rather drag even Rito along with her to avoid being alone with Tearju.
The Lunatique is the artificial intelligence in Yami's space ship which is also where she lives. Lunatique is very loyal to Yami as he always calls her "master" and shows some concern for her. Yami states that he is like an old friend who helps her escape when she is on the verge of life and death. She eventually told him to refer to her as "Yami-chan".
Nemesis is also a transformation weapon like Yami and Mea. Nemesis and Yami seem to know each other, though they first meet each other in Darkness Chapters 23-24. Nemesis originally planned for Yami to return to her old life as an assassin, starting with finishing what she started when first coming to Earth which is killing Rito. However, because Rito can change the way a transformation weapon acts, Nemesis decided to observe how things progress, first by approaching Rito herself.
Nemesis says that one day, Yami will inevitably once again become an emotionless killer when she uses a hidden ability known as "Darkness", which she describes will bring about chaos and destruction to the universe. After Yami's Darkness transformation is defeated by Rito, it is now unknown if Nemesis has anymore plans with Yami.
Yami finds the principal annoying due to him constantly sexually harassing her. When he harasses her, she instantly beats him, which he seems to enjoy.