She has a strong attraction towards Rito (which is similar to Lala's but more lustful). She has stated how his facial expressions are lovely, letting him touch her tail, and even becomes more aroused when Nana refers to him as a "beast". At one point, she felt jealous of Lala when Rito's hand was stuck to her tail.
Momo explains that she first noticed Rito when the latter desperately tried to save Celine, whom he had considered as family despite the inability to communicate with plants. Her affections developed further when Rito pretended to be Peke for her sake, so that she would not get into trouble for the mistake she caused. She also really loves his lustful nature, which he unknowingly shows when he's asleep (To Love-Ru Darkness Ch.14.5). She, like most girls, eventually falls in love with him though unlike the other girls she sexually desires him as well.
In To Love-Ru Darkness, Momo still seeks Rito's love, but unwilling to cause conflict between her sister or Haruna, Momo realizes that if Rito becomes the next king of Deviluke, Earth laws will not apply to him, allowing him to create a harem where she can display her affections openly, and make Lala and the rest of their female friends happy at the same time. After figuring this, Momo starts to develop the Harem Plan.
Rito shows a great deal of care for Momo and her siblings, like when he asked her what kind of person their mother is and if she and her sisters feel lonely not being able to see their mother, she answers no, saying that she has her sisters and Rito and if possible she wants to be with him from now on.
Despite her clear lust and love for Rito when she had the chance to take things further with him in the Gym Cupboard (To Love-Ru Darkness Ch. 17) she grew too scared to continue and recoiled from him. This has become a point of regret and shame in Momo's mind, believing that she lacks the resolve to complete the Harem Plan. However, she has stated she's "beyond that", and currently makes regular attempts to canoodle and nuzzle with him for fun so as to help him get used to the female form; despite overwhelming and annoying him the majority of the time in doing so.
She has shown a devoted caring for his well-being and happiness, constantly standing up for him, and even getting angry towards/threatening those who intend to harm or mistreat him (including her own twin sister), and normally partakes in her sexual advances towards him when she feels it can/will be 'enjoyable' for the both of them, even when it winds up only being fully enjoyable for her. And while her perverted nature does cause him some problems, it's clear that a large portion of this is due to her wanting to love him to the fullest of her abilities, as well as bringing out the pleasing "carnivore" within him for everyone in the harem plan's sexual benefit.
Momo has shown very high respects for her sister, calling her "onee-sama". Momo's respect was also shown when she did not try to steal Rito from her, and instead decided to be content with being a lover. However, Momo has since found a compromise so that her sister would not be heartbroken, and Momo can still be happy. She hopes that when Rito marries Lala, he will be able to marry other girls, which Lala perfectly approves of.
Momo encourages Lala to be more appealing to Rito, especially since she will need to be his legal wife in order to make the Harem Plan possible. She tries to teach Lala some of the more mature aspects of romantic relationships by using her dating sim games as examples (Though she is unable to show her anything beyond kissing, not willing to taint her sister's innocense just yet).
Like any siblings, Nana and Momo often squabble over trivial things, such as Nana's chest, which Momo doesn't hesitate to tease her about. Despite constantly fighting with her twin sister, they both care for each other. She is even including Nana in her "Harem Plan".
In To Love-Ru, they are often seen together, and are often doing things together, such as planning the virtual reality games, and running away from their studies. 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, Momo concentrates on her Harem Plan that was originally proposed by Lala.
She is what people would refer to as a 'tsundere,' while also having a small chest. This makes her have a breast complex, which Momo loves to tease her about.
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 Momo and her sisters, she is the person they admire the most.
Momo has stated that despite being busy most of the time, their mother still makes time to call them (To-Love-Ru Darkness Chapter 23). She finally makes time to visit her daughters after hearing that Lala became small after overusing her powers.
Although she at first welcomed Lala's little sisters into her home, Mikan and Momo's relationship has grown humorously adversarial when Rito is involved. Strangely, while she doesn't mind the childish Lala clinging onto Rito and sleeping naked next to him, Mikan has a particular dislike of Momo's more mature doting and more sexual advances with Rito. Mikan has become something of an obstacle to Momo, barring Momo's access to Rito and guarding Rito's room at night to prevent Momo from sneaking in. She even becomes uneasy when she is forced to leave them alone together at home, and she would rush straight home to prevent Momo from doing anything to Rito.
Interestingly, Momo's existence in Mikan's life has become similar to what Lala's existence was like to Rito. Lala adds new dynamic to Rito's relationship with Haruna, and while none of them hate each other, Rito does put up with much abuse from Lala's inventions and eccentricities. And while Momo plays a role in Mikan's relationship with Rito, her eccentricities have also become the bane of Mikan's normal life. She "helped" Mikan get together with Rito while Mikan was under the influence of Celine's pollen. And she used Lala's invention to turn Rito into panties for Mikan to unknowingly wear so that Rito can find out what's bothering her at school. Mikan states that she can't invite any of her friends over because of all the crazy events happening at her house, especially because of her clumsy brother and the ever scheming Momo.
For the most part, they do get along fine and would hang out together with Nana. They also share some perverted interests such as when Momo tries to bring Rito and Run closer together, they both get excited about what ecchi moment is going to happen with them, and they both enjoy licking on Rito's (as Riko) female body.
Mea can get on Momo's nerves when she mischievously interferes with her relationship with Rito and the Harem Plan, especially talking about it around other people. Ever since she learned about Momo's real self and her Harem Plan, Mea would occasionally entertain herself by annoying Momo and provoking her anger just to check out her bloodlust, which Mea finds surprising and very impressive.
Momo and Nemesis are shown to have a rivalry towards each other, especially involving Rito, like when Nemesis had him feed her and massage her in front of Momo, who was insatiably jealous. Out of all the girls around Rito, she wants Nemesis to keep away from him. Another reason for their rivalry is that Nemesis wants to have Rito all to herself as her pet servant, thus Momo would be unable to be with him or complete his harem.