In 2008, she returned to entertainment as an actress, appearing in Kung Fu Chefs (2009), Ju-On: Black Ghost (2009), and Battle of Demons (2009).

On April 15, she, along with Rika Ishikawa, Hitomi Yoshizawa, and Nozomi Tsuji, were inducted into the idol group Morning Musume as its fourth generation.

In August 2004, Kago and Tsuji graduated from Morning Musume together after Mini-Moni began their indefinite hiatus.

[4][3] Under the arrangement of their agency, Kago and Tsuji formed a new pop duo, W,[3] releasing two albums and six singles together.

Before the release of their seventh single and their third album, W3: Faithful, on February 9, 2006, Friday published photos showing Kago smoking.

[7][8] In late March 2007, photos of Kago going to an onsen with a man 18 years her senior and smoking for a third time circulated in the media, further tarnishing her reputation.

[9] Not long after, Yuukichi Kawaguchi, the director of Up-Front Works, issued a statement announcing that she had been dismissed from the agency.

[citation needed] Later that year, Josei Seven published an interview with her mother, revealing that Kago left Japan and started residing in New York City.

[13][better source needed] Kago made a well-publicized return to the entertainment industry in 2008[14] with plans of pursuing an acting career.

On June 24, 2009, she released her first solo single "No HesitAtIon" [sic] on independent record label In Da Groove.

Since beginning a relationship with Ando, Kago cancelled several jobs at the last minute, causing her agency to suspend her activities.

As a response, in 2011, Kazuyuki Ito, president of Mainstream (an associate of R&A Promotions), declared that the agency planned on suing for ¥100 million in damages for contract violations.

[24] Although Kago was eventually successful in doing so,[25] in August 2015, her contract with her agency expired,[26] though she continued activities with Girls Beat!!

Manga artist Arina Tanemura used her hairstyle as inspiration for Mitsuki Koyama, the main character of Full Moon o Sagashite.

"[30][3] Kago and Tsuji shared the world record for the largest hula hoop spun at 4.08 m (13 ft 5 in) in diameter.

[34] Kago began dating Haruhiko Ando, a restaurant owner in Roppongi, in August 2010,[35] who acted as an in-between between her agency and herself.

[38] In the same month, Kago was rushed to a nearby hospital after agency officials found her on the floor of her apartment with cuts to her wrists.

[45] The police issued an arrest warrant for Ando in October 2014 on suspicion of loaning money at illegal interest rates.

On June 9, Ando was arrested on domestic violence charges stemming from an incident on May 12 where he allegedly shoved and kicked Kago in their home in Roppongi, leaving her with injuries that took ten days to heal.