A childhood friend of Lala, Ren is infamously known for wearing girls clothing during his play dates with Lala when he was a child. He arrives on Earth to win Lala by proving he is "man enough" for her, but becomes jealous of her relationship with Rito. Due to his pursuit of her, he accidentally gives Rito his first kiss. While Ren is furious, it has an unexpected consequence towards his "alter ego". As Ren is from planet Memorze (メモルゼ,Memoruze), he can change gender in mind and body upon triggering a specific stimulus. In his case, a sneeze. As a girl, Run (ルン,Run) is in love with Rito, due to the kiss, and blames Lala for her misfortunes, while Ren is still in competition, albeit one-sided, with Rito for Lala's hand in marriage. Currently, Run is a famous idol, having released several albums. She becomes friends with Kyouko after Run played an antagonist in her show and now are often seen together, even releasing a few albums together.
In the anime, while Ren appears a few times, Run is the dominant personality of the two, who is more serious about her feelings towards Rito to the point of stalking him, even taking a simple misunderstanding from him into her believing all men are never serious about her feelings. In the manga, Ren appears at first as the main personality. However, later on Run becomes the dominant personality (like in the anime) with Ren usually showing up for comedic effect, or when Ren does show up he inevitably changes into Run. In To Love-Ru Troubles, it is revealed they are able to communicate with each other.