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A friend of Lala's since childhood. Whenever he sneezes he becomes a girl named Run.
Ren, unlike his alter ego Run, is obsessed with being the ultimate man, inspired by the movie "Rocky Balboa". Although this is pretty much ignored in the manga, the anime devotes an entire episode to Ren's quest to "become more manly". Involving things such as boxing fights, battling street gangs, fighting the triads, insane excersise routines... he needen't have bothered. Lala never pays attention to him anyway.
Ren hates Rito as his rival, ironic seeing as his alter ego is head over heels in love with him. An... shall we say, humiliating moment for both Rito and Ren occurs in both anime and the manga. Risa drops a challenge that "the first guy to kiss Lala will be her man". Naturally, this never happens, instead when Ren rushes forward to kiss Lala, Rito rushes foreward to stop him... Ren ends up kissing Rito. Both are suitably disgusted by the incident but Lala (air head that she is) takes it that the two of them are now friends.
When Ren is the active persona (and vice versa) he is shown to have the ability to have internal conversations with Run. They often argue over whose turn it is to be the active one.