The rapper and Instagram star known as Tekashi 6ix9ine has pleaded guilty to charges stemming from his role in a violent drug-trafficking gang and was cooperating with federal prosecutors, according to newly unsealed court records. The rapper, whose legal name is Daniel Hernandez, admitted at a federal court hearing in Manhattan on Jan. 23 to being a member of the Nine Trey Gangsta Bloods, and said he had helped other gang members rob people at gunpoint. In one case, he said, “I helped members of Nine Trey attempt to kill a rival gang member.” Mr. Hernandez added, “I did this to maintain or increase my own standing in Nine Trey.” He also said the gang engaged in “shooting at people, robbing people and, at times, drug trafficking.” Mr. Hernandez said he apologized to the judge and “to anyone who was hurt, to my family, friends and fans for what I have done and who I have let down.” Mr. Hernandez remained incarcerated, and it was unclear Friday when he will be sentenced or how his cooperation might affect his prison sentence. At the plea hearing, a prosecutor said that Mr. Hernandez’s cooperation was likely to reveal the criminal activities of other people and that he may eventually need to be placed in the witness protection program. Mr. Hernandez, 22, was charged in November along with several other alleged members and associates of the Nine Trey gang on racketeering conspiracy and firearms charges, a federal indictment shows. The gang “wreaked havoc on New York City, engaging in brazen acts of violence,” Geoffrey S. Berman, the United States attorney for the Southern District of New York, said when charges were announced. In one case last April, prosecutors said, the gang’s members held up a rival gang member at gunpoint near Times Square. Mr. Hernandez helped to plan and direct the robbery, and filmed it from a car, prosecutors said. Later that month, the government said, a member of the gang shot at a rival inside Barclays Center, where Mr. Hernandez was scheduled to perform. Though no one was hit, he later posted a video of himself and co-defendants bragging about the shooting, the prosecutors said. At the time, Lance Lazzaro, a lawyer for Mr. Hernandez, pronounced him “completely innocent” of the charges, portraying him as a victim and noting that the F.B.I. had warned his client of threats made against him. On Friday, Dawn M. Florio, another of Mr. Hernandez’s lawyers, declined to comment on his decision to plead guilty and cooperate, but added that “every effort” was being made “to protect him and his family and his loved ones, for their safety.” Prosecutors declined to comment on the case. In court last week, Mr. Hernandez pleaded guilty to racketeering conspiracy and eight other charges, for which he could face a minimum of 47 years in prison and a maximum sentence of life imprisonment. If he cooperates successfully, the government has agreed to write to the judge, seeking a sentence below the minimum, a prosecutor said. Mr. Hernandez has spent the better part of his adult life crafting a viral internet personality. His brash stunts — which included dying his hair rainbow colors and wearing clothing emblazoned with obscenities — made him a near-instant superstar on Instagram and other platforms. But his meteoric career trajectory as a rapper was unusual. Mr. Hernandez only began pivoting to music after he had become well-known on the internet for his obnoxious antics. He appears to have sought gang affiliation to bolster his credibility among some hip-hop fans. Judge Paul A. Engelmayer ordered the transcript and other records of Mr. Hernandez’s guilty plea temporarily sealed after a prosecutor cited several pending arrests and “safety concerns” with respect to Mr. Hernandez and his family. His plea and cooperation deal may help him obtain a reduced sentence, but it could also be the end of his rap alter ego. Cooperating with the police or federal investigators can be risky in a rap genre that has historically pitted itself against law enforcement, and for him, the stakes may even be higher. For years, his star power stemmed from cultivating a public persona as an outlaw, a self-described “supervillain.” A formal cooperation deal with the authorities undercuts that persona, and he is now labeled a “snitch.”