Luka Urushibara

Ruka Urushibara (漆原るか, Urushibara Ruka) is a close friend of Mayuri Shiina and so-called protégé of Rintarou Okabe. While feminine in appearance and personality, Ruka is a male.

Mayuri is his classmate, who often attempts to coerce him into cosplaying (without much success).

Synopsis
Ruka was first introduced as an extremely feminine individual. Okabe expressed his confusion over this matter quite a few times throughout the game, and at the near beginning of the anime.

After learning about Okabe's experiments, Ruka tells the Future Gadget Lab members a Japanese urban legend: if a pregnant woman eats more meat during her pregnancy, 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 in order to send multiple D-mails to Rukako's pregnant mother. This will cause Rukako's mother to believe that all the D-mails (including the original one sent by Rukako) to be spam, ignores them, and continuing on eating more meat, and thus giving birth to a male Ruka. Because Okabe doesn't want to turn Rukako back in a guy, he doesn't want to wipe out any memories anymore, 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, leap back in time to give Mayuri at least some happiness. The plan was a success, as in that timeline Rukako, Mayuri and Okabe went to the convention together but left Mayuri's fate ultimately unchanged. The scene closes with Ruka producing the pictures they took together with Mayuri at the convention; with Okabe at peace now that he has given Mayuri some happiness. After the credits, Okabe is shown to have fathered a child with Ruka.