Tearju is Yami's creator and DNA origin, who named her Eve. Before Yami's life as an assassin, Tearju treated her like a little sister who was always happy and child-like. She would always look up to Tearju, even now calling her one of the smartest beings there is and even eating badly made food she cooked. Tearju planned on escaping the organization with Yami, but was unable to, thus forcing her to leave Yami behind with the people who forced her to become a weapon.
During their first encounter after being separated for a long time, Yami states that she doesn't blame her for leaving her behind and says that she's glad that Tearju is okay. Currently, Tearju hopes to become closer to Yami like before but Yami usually avoids Tearju unless meeting each other is needed.
Since Tearju's arrival, Yami refused to attend classes and wear her school uniform, which Tearju deeply wanted to see. In To-Love-Ru Darkness Chapter 30, Tearju becomes the homeroom teacher for Nana, Momo, Mea and Yami's class. Yami reveals that she still cares about Tearju, but is too afraid to face her after having become assassin, but with some persuasion from Rito and, surprisingly, Mea, Yami returns to school and attends class with her uniform, which made Tearju very happy.
Yami is also protective of Tearju, especially against Rito's bad habits.
Tearju became Rito's temporary homeroom teacher in To Love-Ru Darkness (after Honekawa-Sensei became absent with terrible back pain). She sees Rito as a close friend of Yami's, and has even shown signs of supporting Yami's hidden feelings towards him. She trusts him and understands that he is a very kind person, and even points out that it's like him to notice that something is troubling someone. She might show a bit of understanding towards his clumsiness as she is never mad at him for it, (even when a perverted incident occured moments after they had first met), and she can also be clumsy herself, as well as shares his trait of being prone to bad luck.
Yami, at first, attempted to remain distant from Tearju after her arrival as a teacher in their school. But after Yami opened up to him about her past with Tearju, Rito became eager to help them reunite with each other so that they may restore the life and happiness they once had together. In Chapter 30, when Tearju becomes Yami's homeroom teacher, Rito, with help from Mea, successfully persuaded Yami to return to school, in the hopes that she and Tearju might become close to each other again.
Mea is another transformation weapon created by Tearju, but not created from her DNA as Yami is. Because, due to unrevealed reasons, Tearju was not there when Mea awakened, and Mea was all alone until meeting Nemesis, Tearju and Mea had no close relationship to start with.
Mea sees Tearju as a "barrier" for her mission to return Yami to being the cold assassin she once was and wants her out of the way for that. However, since Mea is sort of Yami's sister, Tearju considers Mea to be a sister to her as well, and hopes that Mea follows Yami's example by embracing her emotions and bonds, to which Mea rebuffs under her conviction that she is a weapon.
After Mea has reconciled with Nana and become more open to relationships, Mea is still uninterested in having any relationship with Tearju. In chapter 30, Mea still doesn't care about the idea of Tearju and Yami reuniting, but after thinking about what Rito said about how she and Yami are like little sisters to Tearju and how family is important, she surprisingly helps him persuade Yami to come to school as a student again and become closer to Tearju. But Mea still maintains her reluctance to also become close to Tearju herself.
Tearju and Mikado have been friends since they were in school together. They went their separate ways some time before Tearju was recruited into the suspicious organization where she would eventually create Eve (Yami) and Mea.
In To Love-Ru Darkness, Mikado eventually reunited with Tearju to tell her how Eve is doing on Earth. It was because of Mikado that Tearju came to Earth and became Rito's substitute teacher and the new homeroom teacher for Nana, Momo, Mea, and Yami's class.
Tearju seems to know a few things about Nemesis and the organization that created her but doesn't seem to know her in person. But Nemesis knows her quite well and said that Tearju is the "barrier" to Yami from returning to a cold heartless assassin.