Nishikawa debuted as TMR with the release of his first single "Dokusai (Monopolize)" (独裁 -monopolize-) in May 1996. Later that year, his third single "Heart of Sword (Yoake Mae)" (HEART OF SWORD ～夜明け前～) was used as the third ending song for the anime series Rurouni Kenshin, further expanding his fan base. He also contributed six songs to the Mobile Suit Gundam SEED franchise – three for Gundam SEED and three for Gundam SEED Destiny. Nishikawa has guest starred as a minor character in each of those anime series that featured his songs. In 2010, his single "Save the One, Save the All" was used as the ending theme song for the movie Bleach: The Hell Verse. Nishikawa has also had a long-standing affair with Capcom's Sengoku Basara series of video games and media franchise, with several of his songs being used for it, including Crosswise and Flags.
He also contributed with both the opening and the ending theme songs for Marvel Disk Wars: The Avengers, The opening theme song is "Tsuki Yabureru - Time to Smash!" and the ending theme song is called "Thread of Fate" and he also does the voice of Silver Samurai in the series.