"Kizuna" was released only two months after her first single "Pureness". The single was issued in two formats: the limited edition, which includes a bonus CD-ROM featuring a special video message from Ueto as well as a program that allowed fans to receive e-mails from Ueto, and standard CD-only edition. The title track is a mid-tempo ballad and, like "Pureness," it was written and produced by T2ya. It was recorded on September 13, 2002, one day shy of Ueto's seventeenth birthday. At the time, Ueto was busy filming her first feature film, Azumi, and had to learn the lyrics to "Kizuna" on the day of recording.
The song was used in commercials for Karaoke Station by Bandai, starring Ueto herself. Due to her hectic filming schedule at the time, Ueto did not do any televised promotion for the single. The music video for "Kizuna" was shot on the rooftop of a hotel in Osaka. The day of the video shoot, the crew of Azumi were moving locations from Kyoto to Hiroshima, so filming for the video was rushed in order to give Ueto enough time to get back to set the next day. Ueto has stated that the music video for "Kizuna" is one of her personal favorites.
"Kizuna" debuted on the Oricon Daily Singles chart at number 3 and peaked at number 5 on the Oricon Weekly Singles chart, with 32,250 copies sold in its first week. The single charted for eleven weeks and has sold a total of 57,340 copies.