Yousei Teikoku (妖精帝國, Yōsei Teikoku?) (Sometimes misread as Yousei Teikuni) is a Gothic J-Rock group, and part of TEAM FAIRITHM. Their works include the theme songs of the anime shows Magical Pokan, Innocent Venus, and the Mai-HiME videogame soundtrack. They have released one independent album, titled Stigma, and two under the publisher Lantis to which they are currently signed. Fairy Yui (also known as Yui Itsuki) is the vocalist and Takaha Tachibana (who has also worked with similar vocalist Kukui) is the composer of their songs.Two new members have joined Yousei Teikoku some time after the release of the Single "one" and prior to the release of the Single "Baptize". These new members are Nanami who is the Bass Guitarist as well as Relu who is the Drummer.
Through music, Yōsei Teikoku is attempting to revive the Fairy Empire that exists between the human world and the spiritual realm known as Spiritua. The purpose of their music is "to make humans remember the pure heart inside them that believes in fairies, forgotten before we knew it". "Gothic" and "empire" are key words in their musical ideas.