Luka Urushibara

Ruka Urushibara (漆原るか, Urushibara Ruka) is a male who looks strikingly like a female, he's a close friend of Mayuri's who often coerces him into cosplaying (without much success.)

Synopsis

[SPOILER ALERT] Introduced as an extremely feminine boy, Ruka later reveals that he is a transexual, stating that his his heart, he's always felt like a girl in spite of male body. This disqualifies him as a "trap" (animespeak for transvesitite) because Ruka desires the body of the opposite sex and not to just dress like the opposite sex. Ruka tells the Future Gadget Lab members a Japanese urban legend: if a pregnant woman eats more meat, then her baby will be a boy, and if she eats more vegetables, her baby will be a girl. The Future Gadget Lab, although skeptical of whether or not the urban legend is true, lets Ruka send a D-mail back to his pregnant mother's pager, telling her to eat more vegetables. In episode 10, it's revealed that the D-mail has worked and the characters are transferred to a world line where Ruka becomes Rukako. ("-ko" is a common ending exclusive to female names, and therefore, would mean in that world line, Ruka has become a biological girl.) This causes an extremely awkward situation in which Okabe does not realize this and gropes Rukako's crotch in order to prove to the Future Gadget Lab that Rukako is male. He quickly learns that he's been moved to a world line where Rukako is a girl and that he's been accidentally, sexually  harassing her.

Ruka's Ending
In this world line, Okabe decides not to send the D-mail that would make Ruka return to being a boy again. He convinces himself that Mayuri will still die no matter what he does and he doesn't want to sacrifice Rukako's happiness. Rukako confesses to Okabe and swears to give him (Okabe) his mother's pager number on the condition that the two go on a date together once. The two's date is deeply awkward and contrived, so much that Rukako cuts it off abruptly, leaving Okabe to guilt over it. He realizes that it wasn't truly a date because he was trying too hard to impress her by not being himself, and rushes over to the shrine where the two spend genuine bonding time together. After which, Rukako gives Okabe his mother's pager number, and the Future Gadget Lab send multiple D-mails to Rukako's pregnant mother. This causes Rukako's mother to believe that all the D-mails (including the original one sent by Rukako) to be spam, ignores them, and continue on eating more meat, and thus giving birth to a male Ruka. The timeline goes on and Mayuri still dies. Realizing the devastation she has wrought, Ruka feels guilty about Mayuri's death. Remembering that Mayuri wanted Rukako to cosplay in the next convention, Rukako with the help of Okabe, travels in time to give Mayuri at least some happiness. The plan is a success, as in that timeline Rukako, Mayuri and Okabe go to the convention together but Mayuri still dies. However, Okabe is okay with it, glad that he could give Mayuri some happiness.