t.A.T.u. Singer Lena Katina Sued -- I Resurrected Kissing Duo ... Pay Me $20,000,000

Singing/kissing Russian schoolgirl duo t.A.T.u. would be FORGOTTEN if not for one industrious manager -- so says the manager, who claims he's responsible for bringing the girls back from obscurity ... and now, he's suing for more than $20,000,000.

The group's popularity declined in 2003 -- following the massive hit "All the Things She Said" -- and Lena Katina (red hair) and Julia Volkova (black hair) eventually broke up years later, but a manager named Tristram Buckley insists he's responsible for bringing them back together.

Buckley has sued t.A.T.u. member Lena Katina (the redhead) as well as the group's founders, claiming he managed Lena from March 2012 - March 2013, and completely turned around her dwindling career. Buckley says his hard work culminated last year when t.A.T.u. reunited.

When he met Lena, Buckley says the singer was depressed and directionless -- her solo career was losing money, and she was "arrogant, abrasive, confrontational and rude" to fans -- and he rebuilt her image from the ground up.

But Buckley claims he was hung out to dry ... wrongfully terminated after just one year, and never paid for his services.

He's now suing for fraud, breach of contract, defamation and more -- demanding more than $20,000,000 in damages. Calls to Lena's camp were not returned.