Xジェンダー (x-jendā) is the most common term I’ve found in my research thus far on an identity that is neither man nor woman in Japan, comparable to the English-language terms genderqueer or the more neutral term non-binary. Collected here are a list of some of the most informative and engaging sites I could find on the subject, some provided with annotations.
Intersections: An Introduction to X-Jendā: Examining a New Gender Identity in Japan: This was one of the only pieces I could find in English about the term, so I would heartily recommend checking it out for a comprehensive look that explains the identity within the framework of Japanese terminology and culture and outlines a history of the word’s usage and other similar terms.
クイアな必然 (Kuia na Hitsuzen; A Queer Inevitability): This is the blog of Nosuma, who identifies as Xジェンダー and as XTX specifically; compare with uses of FTM, MTF. Also, please note that the terms FTM and MTF are presently very common in discourse around gender identity in Japan, even more so than transgender man and transgender woman. They are also an artist and have designed a series of shirts defining Xジェンダー at their Atelier Saranse shop.
虹色ろんど (Nijiro Rondo; Iridescence Rondo): This is a blog by Seiji, who is quite humorous and often adds illustrations to his blog articles. Seiji describes himself as identifying as FTM in a more lengthy biographical description and オナベ野郎 (Onabe Yarou; a Male-Identified Rascal) in the fill-in box for their gender identity – yes, although you’re required to pick male or female to start, Ameba lets you fill your gender in with text afterward! Seiji has done blog posts defining gender terms, such as this piece, a Sexual Minority Glossary, which defines such terms as coming out, neko and tachi (terms often used in the lesbian community to denote passiveness or activeness in sexual intercourse), danāzu (trans men who are attracted to other trans men), femme, and 熊系 (kumakei, bear system, preference for the bear type in the gay community), and this entry which explains the difference between FTM, FTX, and related terms.
ちぃのGID-MtFの４ｺﾏﾌﾞﾛｸﾞ(Chii no GiD-MtF no 4 Koma Burogu; Chii’s 4-Frame Comic GiD-MtF Blog): Although focusing on MTF identity typically, this particular comic and article discusses Xジェンダー: [4ｺﾏ]☆Ｘジェンダー☆. Chii describes the differences between 中性 (chūsei) and 両性 (ryōsei), wanting to identify between man and woman (男と女の中間でありたい！) and wanting to identify as both man and woman (男女どちらの性でもありたい！) respectively and relates an anecdote about an Ｘジェンダー person in junior high who was uncomfortable wearing their high school uniform because it was in a girls’ style.
Xジェンダー Groups on Ameba: Groups available range from the large, with over 15,000 members such as 性別?そんなの知りません!(笑) (Seibetsu? Son’na no shirimasen! (Wara); Gender identity? I don’t know what that is! (Laughs)) that encompass a variety of gender identities, including binary associated ones, to the small Xジェンダーの憩いのお部屋 (X-jendā no Ikoi no Oheya; An X-Gender Room for Relaxation), which I should note is also managed by Nosuma (mentioned above). Member blogs are listed under メンバーのブログ within each group and include many entries by people who identify outside of the binary. See also the Xジェンダー Group on Mixi.
Euphoria: ～Xｼﾞｪﾝﾀﾞ-中心の総合情報サイト～ (Euphoria: X-jendā Chūshin no Sōgō Jōhō Saito; Euphoria: Comprehensive Information Site on X-Gender): This site includes a glossary of terms concerning gender, sex, and sexuality and detailed information on FTX identity.
Xが集まれる場所 (X ga Atsumaru Basho; X Gathering Place): Relatively new site gathering information about Xジェンダー and related terms, including books and links.
Xジェンダー＠Wiki (X-Gender Wiki)