The Japanese manga series To Love Ru and its sequel To Love Ru Darkness feature an extensive cast of characters created by author Saki Hasemi and illustrator Kentaro Yabuki. After high school student Rito Yuuki accidentally gets engaged to Lala Satalin Deviluke, the runaway crown princess of the alien planet Deviluke, he reluctantly helps her transition to life on Earth, while gradually befriending his dream girl, Haruna Sairenji. Along the way, Rito meets and gets entangled with a colorful cast of other girls (both humans and aliens alike), such as the uptight, high-strung Yui Kotegawa, the sex-switching alien Run/Ren, and the queen bee of their school, Saki Tenjouin. At the same time, Rito must fight off Lala's antagonistic alien suitors, one of whom sends the assassin Golden Darkness to kill him.

Hasemi said that the latter half of the manga features a lot more quirky and unique characters because, the newer the character, the harder he and Yabuki had to work to establish their personality. Yabuki said that by the latter half of the series, they were treating all the female characters as main heroines. Many of the characters also make a cameo appearance in the manga version of Mayoi Neko Overrun!, which Yabuki illustrated.

Rito Yuuki (結城 梨斗, Yūki Rito) is a 15-year-old high school student at Sainan High and the main protagonist of the series. His younger sister, Mikan, comments he is very good at the most "pointless things", like crane and festival games such as catching fish and squirt guns. Rito is an accomplished florist, gardener, and horticulturalist. He is extremely kind, selfless, caring, and thoughtful, having a lot of respect for women, though mostly by intention, and not so much by success. It is only by accident that he encounters Lala Satalin Deviluke, and despite his enduring feelings for Haruna Sairenji, Rito develops deeper affection towards Lala throughout the story, but is unsure if what he feels is love. At the end of To Love Ru, he confesses his love to Lala but tells her he loves Haruna more. However, she misunderstands and is happy at the prospect of him being married to both of them. In contrast, polygamy is illegal in Japan. Once he is king of the galaxy, the laws of Earth no longer apply. By the end of To Love Ru Darkness, Rito earns Haruna's mutual feelings; in addition to Lala, Run, Yami, and Momo. The ending suggests that his love troubles will persist indefinitely.

Due to his shyness around girls, he is very clumsy and has great difficulty maintaining his composure around any sort of stimulating situation, which leads him to accidentally molest or grope the many girls in the series. Due to the otherworldly circumstances around him, Rito is always put, along with many different girls, in any and each type of awkward, intimate, and embarrassing situations, such as when he and Haruna were once teleported naked together to the basement of Mikado's medical clinic because of one of Lala's faulty inventions, or when Lala used the same invention to send them naked to the school. In To Love Ru Darkness, his clumsiness becomes even more frequent and violating as well as strangely inventive and surreal. He has shown to be able to accomplish difficult feats, such as reaching his hands or various objects into the clothing and undergarments of girls or partially stripped off their clothing, and also grabbed several private parts of several girls all at once, all of which were purely by accident. In a later chapter of Darkness, Mikado conducts some experiments to gather some data and later interviews Rito. Mikado diagnoses that Rito's frequent godly indecency and unwitting perverted acts manifest because of the way Rito extremely restrained himself with the girls since he never sought the opportunity to get intimate with them and wanted to treat them with kindness, causing his "libido" to accumulate and release in form of accidentally groping girls' bodies and all other perverted stuff happening so far, calling it the "Cataplectic Indecency Syndrome". She also believes that the more he suppresses it, the more extreme and unbelievable the accidents will be. Mikado calls this not a disease, but rather a talent that is beyond an average human. During the first manga series, continuing into Darkness (and present in some episodes of Motto To Love Ru), Rito also seems to suffer from Sexsomnia. This can most often be observed with Momo being on the receiving end of it.

When he is turned into a girl, Rito takes on the persona of Riko Yuusaki (夕崎 梨子, Yūsaki Riko). She is a beautiful girl (with a physical appearance resembling a tomboy) who gets much attention from men, such as Motemitsu, the principal, and even Saruyama, Rito's best friend. Later, Saruyama falls in love with Riko and asks Lala to invite her for a date; out of pity, she forcefully changes Rito into Riko so Saruyama can go on the date. After that, Riko only appears for a short number of times on rare occasions. Her last appearance was in Chapter 74 of Darkness manga when Nemesis changed her sex through boy-girl change-kun.

Saki Hasemi and Kentaro Yabuki initially imagined To Love Ru as mainly a comedy featuring Rito and Lala, with Rito only longing for Haruna. But as they had more meetings, this changed to Haruna also being a featured character with feelings for Rito, to emphasize the love triangle. They took care to make sure that Rito was likeable and that his actions were not unpleasant. Hasemi said that because it ran in a shōnen magazine, if boys did not like and support the protagonist, then drawing cute girls would be meaningless. Riko, Rito's female form, was thought up as something special for the 100th chapter and to celebrate two years of serialization. They never intended for her to reappear, but she did so very quickly because she was shockingly well received. Rito is one of the most popular characters in the series, coming in fifth place in the manga's first character popularity poll.

Lala Satalin Deviluke (ララ・サタリン・デビルーク, Rara Satarin Debirūku) is the first princess of Deviluke, whose family rules the Milky Way galaxy. As such, her hand in marriage is sought from across the galaxy as her husband will be successor to the throne. A rather childish girl, she is naïve regarding the culture and customs on Earth. Lala initially runs away from home, as she does not wish to marry any of her suitors, and randomly ends up naked in Rito's bathtub due to one of her inventions. Upon arriving on Earth, Lala pretends to love Rito to keep herself from getting married but really does fall in love with him after she misinterprets a statement that Rito shouts that was meant for Haruna. Although Lala appears to be somewhat airheaded, she is well known on Deviluke for her genius-level intellect and enjoys making all manners of inventions, often prompting some disastrous effect or another.

Haruna Sairenji (西連寺 春菜, Sairenji Haruna) is Rito's classmate, and the girl of his affections. Unbeknownst to Rito, Haruna has similar feelings for him, having been attracted to his kind, gentle nature since junior high. Despite her growing love for the boy (which she believes to be unrequited), she usually suppresses her feelings and cannot confess to him because of her friendship with Lala. However, with some encouragement and knowing that she cannot continue to hide her feelings, Haruna eventually tells Lala of her ordeal. Lala warmly accepts Haruna as her rival for Rito's heart, while still remaining friends.