She joined the brilliant green in 1995 after bassist Shunsaku Okuda and guitarist Ryo Matsui heard her sing in a karaoke contest. As the band's lyricist, she sings and writes songs in both Japanese and English. Since the release of their 2002 album THE WINTER ALBUM, the brilliant green have been on hiatus, and in 2003, Kawase announced her marriage to bassist Shunsaku Okuda.
In 2001, Tomoko launched her solo career under the name of Tommy february6, named from a combination of her nickname and birthdate. Tommy february6's sound is inspired by and derived from British and American 1980s synth-heavy pop.
Kawase originally intended to create a character that was far from innocent, although cute in both sound and appearance. Tommy february6 was intended to be someone true to her feelings who did as she wanted. She is known for her geeky look, cheerleader themes and innocent personality as well as her love of flasks. Although the February Project was not meant to be very long-lived, it gained a large amount of attention.
The original concept started to disappoint Kawase and after a performance as Tommy february6 in 2003, she took off her glasses and donned a much more alternative look to create Tommy heavenly6. This character had a modern rock sound.
According to Kawase, Tommy heavenly6 was born from one of Tommy february6’s dreams in which she embraces all the elements of her personality she had previously been repressing. In the promotional video for Tommy heavenly6's first single, Wait Till I Can Dream, Tommy february6 drinks from a hip flask and falls asleep to be woken by Tommy heavenly6. Musically, she's the opposite of Tommy february6 and is is meant to be the dark side and the alter-ego of Tommy february6. The character is more sophisticated and classy and obsessed with her image.
Kawase chose to keep the two characters' careers separate and during 2003 and 2004, Tommy heavenly6 released only two singles, whilst Tommy february6 went on to release her second full album, Tommy airline. It wasn’t until August 2005 that Tommy heavenly6 released her third single, shortly followed by her debut album.
In addition to her solo singing career, Kawase has produced and written songs for the idol group Tommy Angels, as well as making a protégé of former soloist Asuka Hinoi. Kawase has also appeared as the voice of Pikki in the anime series Piroppo, briefly modelled for Italian fashion house Fiorucci and hosted Break Down, an MTV show.
In October 2002, Tommy february6 released a photo and journal book titled Tommy february6 + Hawaii.
In October 2005, an exclusive Tommy february6 Blythe doll was released.