Rodgers has accused Kelly of locking her up and filming non-consensual sex.
The six-hour documentary "Surviving R. Kelly", which aired earlier this month on USA cable channel Lifetime, includes allegations from multiple women who accuse Kelly of sexual misconduct, sometimes with minors.
The women's rights attorney made the announcement at a press conference alongside Faith Rodgers, who previous year sued the singer for "mentally, sexually and verbally" abusing he during a relationship when she was 19.
At a press conference on Monday, Allred revealed that she is representing a woman named Faith Rodgers who said that Kelly had mistreated her.
In the letter, a person identifying himself as R. Kelly warned a lawyer for the woman, Faith Rodgers, that if she persisted with the suit, she would be "subjected to public opinion".
On January 11, Kelly's attorney Steve Greenberg denied all allegations of sexual misconduct made in "Surviving R. Kelly".
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It reads she should stop if she "really cares about her own reputation".
Appearing on Good Morning America last week and discussing R Kelly's rumoured marriage to Aaliyah when she was 15, his lawyer Steven Greenberg said, "My understanding is that she did not claim to be 15 and, in order to get married, she had to lie about her age".
When pressed further by the outlet if he thought people were "rewriting" history a bit, the "Better Now" hitmaker explained that he thought Kelly "had done a lot of fucked up shit".
Rodgers, now 21, gave a brief statement, saying, "No woman should be victim shamed, harassed or retaliated against because she asserted her rights and spoke her truth".
Besides tracking his life, career and creation of Miramax and The Weinstein Company, it also documents the shocking allegations that emerged on 4 October 2017 in an expose in The New York Times detailing his decades of abuse, and his fall from grace. Color-of-Law is NOT law.
The R&B singer has been ordered to pay $173,000 in back rent and court fees by Monday, or he'll be evicted from the warehouse.