Momo first made an appearance together with Nana in Chapter 97 of the To LOVE-Ru manga. They were responsible for the Trouble Quest, a virtual reality RPG game that they programmed to observe Lala's friends. After seeing that Lala had great friends surrounding her, they returned to Deviluke. A few chapters later, they went back to Earth after escaping from Deviluke and their studies. However, they forgot that Zastin was also staying on Earth, who was instructed by their father to bring the them back home. It was revealed that Momo and Nana also have D-Dials, but instead of gadgets, they were able to summon plants and animals, respectively. In the end, they defeated Zastin and his assistants, and convinced their father that they're going to stay on Earth with Lala.
During Momo and Nana's stay in the Yuuki household, they created their own house in the attic using space distortion. They also took care of all their necessities to avoid bothering Mikan for cooking, cleaning, etc. However, they still come down sometimes to eat Mikan's cooking.
Throughout the series, Momo gradually developed affectionate feelings for Rito despite him being the fiancé of her sister. Momo would constantly sneak into Rito's room, sometimes even half-naked, revealing Momo's lustful side. Whenever she's asked of the situation, she simply says that she accidentally gets into Rito's room while she's half asleep. Momo has also stated that she knows Rito would not reciprocate her feelings, and that she's content just to be a lover.
In To LOVE-Ru Darkness, Momo organizes the "Harem Plan", that was originally proposed by Lala. This is a plan to make sure that Lala can still marry Rito, and at the same time, Momo's love for Rito would be reciprocated. The first phase of her plan was to get into Sainan High so that she can monitor Rito and the female characters closely. The second phase was to turn Rito into a "carnivore" so that he would take it upon himself to create a harem. To do this, she manipulates the situation so that Rito would end up in a romantic situation with one of the female characters that have feelings for him. Of course, Momo also manipulates situations so she can get a romantic situation with Rito. So far, herself, Lala, Haruna, Nana, Yui, Yami, Run, Rin and even Mikan (despite being his sister) are part of her "Harem Plan", with Mea, Kyouko and even Dr. Tearju under consideration to be part of the plan but, it's still unofficial as to whether or not they'll be part of the plan in the end. As of now, Momo's plan is gradually taking effect as initially, Rito rejects the idea, but is slowly considering it.
She continuously tries to find ways to make every girl around Rito fall for him as part of her Harem Plan. She would sometimes buy love simulation games as part of her research for the plan, which she uses to set the mood for him and a certain girl he's with or even when it's her. She hopes that one day Rito might return her love as well as the love of every girl who has feelings for him.
After her mother, Sephie, comes to visit them, she and her sisters spend time together in the hot springs in Nana's cyber safari park. Momo hopes that her mother would not find out about the harem plan, but after a brief moment with Rito, Sephie quickly figured out that Momo is planning a harem for Rito hoping to be a part of it. Much to Momo's shock, Sephie tells her that she's fine with Momo trying to get what she wants as long as her heart is fine with it.
Ever since her talk with her mother, Momo is expressing doubts about whether she would be satisfied with her harem plan. Most notably, in Darkness chapter 55, when she learns that Rito and Haruna accidentally kissed each other, instead of becoming happy and enthusiastic like she would whenever Rito gets development with the other girls, Momo becomes worried, restless and uneasy. Much to her own surprise, she feels like a loser. Despite this, she pushes Rito to finally confess to Haruna. When Momo watches as Rito and Haruna's attempts to confess to each other once again went nowhere, Momo somehow feels relieved. However, after Lala reaffirms her own love for Rito and satisfaction in receiving love from him despite his love for Haruna, Momo also reaffirms her own satisfaction to receive his love and restores her resolve to resume her Harem Plan.