Keiko began singing, along with many other familiar names (Wakana, Kaori, Yoshida Yuuki, Ito Ayaka), when she became sponsored by Spacecraft (a talent agency in Japan). She performed alongside these other girls in many types of small concerts.

Her recording career began when she first worked with the already famous composer, Yuki Kajiura, on an insert song for the Tsubasa CHRoNiCLE anime. The song, "Kaze no Machi he," (風の街へ) featured a vocal style Yuki Kajiura had not focused much on in the past, an alto range. This made Keiko very unique in comparison to the other few dozen vocalists that were credited alongside Yuki Kajiura, and earned the name FictionJunction KEIKO, an honor only a handful of the collaborations can boast about.

After recording and releasing this song, Keiko teamed up with a friend named Ito Ayaka to form the duo "Itokubo". They recorded a song for a Japanese video game together ("Beautiful Katamari") and did a number of shows for about two years, before they quit in January of 2007.

Sometime in mid-2007, Yuki Kajiura was asked to do the soundtrack for the Kara no Kyoukai series of movies. Being a different sort of project - a series of movies, all with different themes, rather than a single anime series, all connected by the same music - she decided to take a different approach and held auditions for a group she named "Kalafina," who would do the ending themes to each movie. Keiko auditioned and was one of two vocalists to become a part of Kalafina, the other being another FictionJunction vocalist, Wakana.

Kalafina released their first single, "Oblivious," which consisted of the ending songs for the first three Kara no Kyoukai movies. At this point, Sony, the label that Kalafina had signed with, held auditions to add two more members to the group. Out of 30,000 people, Hikaru and Maya joined for the Sprinter/ARIA single. Maya quit shortly thereafter. After releasing various other singles for projects other than Kara no Kyoukai - such as the Lacrimosa single, they released a full album "Seventh Heaven," featuring all 7 Kara no Kyoukai themes and a few extra tracks.

Around the time that Kalafina was getting its start, Yuki Kajiura started performing live concerts. The first concert was held in April of 2008, and featured four vocalists - Wakana, Yuriko Kaida, Kaori, and Keiko - singing various popular songs from the many soundtracks Yuki Kajiura had composed. This concert was so popular that a second concert was held on July 31, 2008, of which a DVD was recorded of. A third concert in December was put on to celebrate the DVD release. Because of the popularity of these concerts, a July 2009 tour was planned, with three concerts instead of just one.

The tour was to celebrate the FictionJunction album "Everlasting Songs". With this album, all of the six FictionJunction vocalists had solos, but they also often provided haromony for the others as well. With the release of the Pandora Hearts OP, "Parallel Hearts," it was announced that instead of having the various FictionJunctions - such as FictionJunction KEIKO - FictionJunction would just be credited as is, and be a collaboration betwen Yuki Kajiura, Keiko, and the other singers, Wakana, Kaori, Asuka, Yuuka, and Yuriko Kaida.

Keiko has left Kalafina as of April 1, 2018. She's still in FictionJunction.

Keiko announced her solo debut via Avex on April 16, 2020.