Yūsaki Riko

Riko Yuusaki is Rito Yuuki's female alter-ego, who is accidentally created through Rito being transformed into a girl by one of Lala's bizarre inventions which she dubbed Pai-Pai Rocket. Later Lala had created another invention called Koro-Koro Danjo-Kun/Change Boy Girl-Kun to change Rito back into Riko anytime.

Appearance
Riko looks greatly similar to Rito as some characters have noticed, she has gold brown eyes and has slightly long straight hair and it's less spiky then Rito's. She is a beautiful girl (with a physical appearance resembling a tomboy) who gets much attention from men, such as Taizou Motemitsu, the principal and even Kenichi Saruyama. Not only is Riko quite attractive to a lot of boys, her looks are even appealing to girls. She has a great bust size which even makes Nana jealous. Haruna and Mikado apparently finds her cute and Momo even mentioned that Riko is still her type even in this form.

In To Love-Ru Darkness, her hair is now all straight and for some reason the back of her hair is noticeably shorter than it was when she appeared in To Love Ru. Her 3 sizes are 85-55.5-84

Personality
Rito still retains his usual personality and behavior but tends to feel uneasy when in this form due to the exaggerated body change, regardless of how many times he turns into Riko. However he tries to pull off a girly personality when around characters who don't know about his/her condition, even if they do know of Riko's true identity, Rito is prone to act in a slightly more female manner in this state. By turning into Riko has a slight understanding of what Run and Ren go through when changing.

Comically, Rito does seem to consciously struggle with the gender he is placed in, such as when teased about it by Mikan (who can't seem help but tease "Her" by reffering to her as her older sister - this does seem to hint, as Momo noted, that Mikan might be something of an "S" around Rito). Riko appears to be quick enough to adapt to keep up a good act and function in a role, such as dating Saruyama or working in Mikado's clinic.

Rito's legendary falling issue is also present in this form, but it can also result in "her" being gropped instead of doing so to others.

Plot
The character is initially created when Lala's device, designed to make a female's figure perfect, blows up when Yami attacks Rito and sends him flying into it. After emerging from this cloud, Rito had been transformed into a female form. At home, Mikan and Lala both teased "her" on the matter, eventually causing her to run away. After being attacked by the Principal and passing out, she was taken in by Saki Tenjouin, until Lala arrives in an untimely manner and returns her to her male form right in front of Saki, Rin and Aya - all of them naked in a bath, and he was subsequently chased out. After that, Riko only appears for short amount of times at rare occasions. While as Riko, it is often stated that she is Rito and Mikan's cousin to avoid suspicion.

Every now and then, someone (either Lala or Momo) would force Rito to change into Riko merely for humor or for illustrations. Sometimes she would appear in Saruyama's love fantasies for such things like Valentine's Day.

Later in To Love Ru, Riko reappeared when Lala tried to get Rito to go on another date with Saruyama (who had fallen in love with "Riko-Chan") out of pity. Through the whole day, Rito is disgusted with Saruyama's obnoxious attempts at romantic approaches. At the end of the date, Riko turns back into Rito and runs off before Saruyama (who was about to kiss him) could see him, only for Rito to be seen by Haruna in girl's clothes. While being chased by Yami due to another perverted mishap, Lala turns Rito into Riko again while he is running away. Riko later ran into Yuu Kotegawa, who protected her from the Principal, where she transformed back into Rito right in front of him. Yuu learns of Rito's predicament with aliens, and is now one of several characters who know that Rito and Riko are the same person.

In To Love Ru Darkness, Lala transformed Rito into Riko just to show Mea Kurosaki a  demonstration of Run/Ren's nature. This marks the second time most of the characters present have seen the form, with Nana coming to envy how Riko's chest is so large and Sairenji coming to consider the form cute. This change forced Rito to leave the school early and return home (especially when Saruyama came in), with Mea and Lala continuing to harass her along the way and even dressing her up in a maid's outfit. When "she" is finally alone, Rito takes a moment to scrutinize his female form, only to be caught by Momo, who takes it as evidence of his fascination with the female body. This meeting eventually leads to Mea returning, as planned, and discovering Momo's true personality. The two girls then reach an agreement to become friends, and even co-operate on the Harem plan, but Mea notes she cannot help recruiting Yami into it. The two then started to "play" with Riko, only to be caught by Mikan right when he turned back.

Riko makes a second reappearance in Darkness, where Momo transforms Rito (just to dress her as a nurse) to help in Mikado's clinic (who took a mischievous liking to her). She accompanied Yami to the basement of said clinic to see what had happened to Tearju, and helped convince Yami to try getting closer to her creator. When it is revealed that Tearju had accidentally activated a dangerous automaton in the basement, Riko and Yami tried dodging it's attempts to "apply first aid" but ran into each other, where in a comical reverse of their usual playaround, Yami ended up groping Riko's chest. After destroying the automaton, Yami found that Rito (completely naked) had turned back to normal and was unconscious. She later called it unfair of him to turn back in that situation, as she caught sight of "the beast" and was forced to dress him, which gave her an intense blush.

Trivia

 * Due to being the female form of Rito and him being Mikan's brother, Riko would technically be Mikan's sister (a fact Mikan exploits for her own amusement by calling her "Onee-chan").
 * With this in mind, it is probably right to consider Riko as Rito's twin fraternal sister, though there is no more alternative personality created.
 * Rito's clumsiness carries over to this form, which ironically resulted in her breasts being gropped by Yami. When Riko called it shameless, Yami asked in bewilderment if "she" should be the one saying so.
 * The same thing roughly happened with Nana as well, where the young princess ended up slapping Riko, due to feeling like she was insinuating that her chest was small.