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
WAKANA
YURIKO KAIDA (Also has solo works as KAiiDA)
YUUKA (solo work as Yuuka Nanri)

Other Utahime (currently not performed as "FictionJunction"):
ASUKA (performed only in "Everlasting Song". Currently Aira Yuuki)

KEIKO and WAKANA are also part of defunct Kalafina with Hikaru.
KAORI is also part of Sound Horizon.