Mr Kavanaugh denied he was "at any such party".
Ramirez said Kavanaugh exposed himself to her during a drinking game in their freshman year at Yale.
Roche also said: "Based on my time with Debbie, I believe her to be unusually honest and straightforward and I can not imagine her making this up. She admits time lapses".
Roche, who declined media interviews, said he he did not witness the 1983 act in question: When Debbie Ramiez said Kavanaugh was standing next to her, laughing, pulling up his trousers, after putting his penis in her face, the New Yorker reported on Sunday.
President Trump continues to support the nomination of Judge Kavanaugh.
"The truth is I've never sexually assaulted anyone, in high school or otherwise", Kavanaugh said on Fox News Channel's "The Story with Martha MacCallum", according to excerpts released by the cable network. The other people alleged to be there don't recall any such thing. As we got to know one another, I discovered that Debbie was very anxious about fitting in.
This follows an allegation from Christine Blasey Ford, who says Kavanaugh sexually assaulted her at a party in 1982, when the two were teenagers.
"Ms. Ramirez is ready to swear to the Federal Bureau of Investigation under penalty of perjury", he said.
With Kavanaugh's confirmation prospects already hanging the balance, the news of a second set of allegations is likely to alarm Republicans who are hoping to push Kavanaugh's nomination through the Senate, where they hold a slim 51-49 majority.
Kavanaugh went even further in the Fox News interview.
Lori A. Ringhand, a law professor at the University of Georgia and an expert on the confirmation process, noted that Felix Frankfurter became the first Supreme Court nominee to testify in public to counter criticism that surfaced during his hearing.
The Supreme Court nominee made it clear - as has the White House - that they're not backing down.
Like Christine Blasey Ford, the professor accusing him of assaulting her when they were teenagers, Ramirez wants the FBI to investigate the incident, and Democratic lawmakers are backing their demands. The two are expected to discuss Rosenstein's role in the Russian Federation investigation, as well as reports claiming Rosenstein suggested secretly taping the president or asking for support from within the Cabinet to impeach Trump.
US President Donald Trump has chose to stand behind his Supreme Court nominee Brett Kavanaugh.
The roadblocks to confirming Brett Kavanaugh as a judge on the United States Supreme Court are mounting.
Kavanaugh denies the women's allegations, calling Ramirez's claim "a smear, plain and simple". He described Kavanaugh as a "great scholar" and a "fine man". Republican leaders and Trump have rejected Democratic demands for the FBI to investigate the allegations.
Trump called Kavanaugh "just a wonderful human being" and suggested that Democrats were skeptical of Ramirez, saying, "They don't believe it themselves".
"As we have stated previously, sexual assault must not be normalized or condoned in any way or by anyone, especially those charged with political leadership", the group said.