Urushibara, Ruka

Second year in Kasensou Private Preparatory Academy, and Mayuri's classmate. Born biologically male. He has the personality of a polite, well-raised and justified kind of beautiful girl.

[Spoiler] His biggest wish is to be born again as a female. [/Spoiler]

He uses honorifics specifically with Rintarou. Doesn't push his opinions much upon others, but when he does he's quite bashful and blushes quite a bit. Because he's quite serious by nature, he simply believes what others say. Thanks to that, he thinks all the ridiculous stuff Rintarou talks about is that of reality. He's a very shy person, and often turns down Mayuri's requests to cosplay.

[Spoiler] He eventually gets his wish and sends a text to his mother's pager, telling her to eat more vegetables. This is said to help increase the chances of a child becoming female. Okabe experienced a shift in the world lines and Ruka is successfully changed into a female. This is all undone eventually in order for Rintarou to save Mayuri. [/Spoiler]