FictionJunction

FictionJunction (フィクションジャンクション) is Japanese composer Yuki Kajiura's solo project. It is not a pseudonym that Kajiura uses when she composes soundtracks. To date, all such collaborations have consisted of a (female) vocalists, aka Utahime, with Kajiura on the keyboard or the piano.

Current Utahime:
KAORI (also has solo works)
KEIKO
YURIKO KAIDA (Also has solo works as KAiiDA)
JOELLE

shuri(as of now, performed only in "fairy game" from Princess Principal)

LINO LEIA (won an audition contest to perform in FictionJunction's new album)
rito (won an audition contest to perform in FictionJunction's new album)

Other Utahime (currently not performed as "FictionJunction"):
ASUKA (performed only in "Everlasting Song". Currently Aira Yuuki)
YUUKA (solo work as Yuuka Nanri)
WAKANA
LiSA (performed only in "from the edge")
ReoNa, Eir Aoi and ASCA will perform SAO's 10th anniversary theme song "Fanfare of the Blue Sky".

KEIKO and WAKANA were also part of defunct Kalafina with Hikaru.
KAORI and JOELLE are also part of Sound_Horizon.