Arianne, her full name Arianne Schreiber, is the South African-born British singer (for details see this post) of two songs in Evangelion. One is Komm, süsser Tod, Come Sweet Death, which was used in The End of Evangelion when Third Impact began. The other song, which wasn't used for unknown reasons, is Everything You've Ever Dreamed, only available on two albums. Both songs were composed by Shiro Sagisu, with lyrics adapted into English by Mike Wyzgowski from Hideaki Anno's original Japanese version.
According to her Facebook page, Arianne was in Japan for 3 days to do recording on Komm Süsser Tod and Everything You've Ever Dreamed. She said it was "amazing". She was also "honored to be a part of Evangelion".
The music she performs in her band is electronica.