NEW BRUNSWICK, New Jersey (Reuters) - The mystery man whose homosexual tryst with Tyler Clementi is at the center of a privacy invasion trial prepared to testify as early as Thursday against the former Rutgers student accused of using a webcam to spy on them in a dorm room.

He was one of the last people to see Clementi, 18, who committed suicide by jumping off the George Washington Bridge on September 22, 2010, just days after learning his Rutgers University roommate Dharun Ravi, now 20, covertly saw them kissing and encouraged others to do so too.

Ravi is not charged in Clementi's suicide, which was widely portrayed as a tragic example of bullying and the toll it too often takes on gay teenagers.

Ravi is charged with 15 counts of invasion of privacy, witness and evidence tampering and bias intimidation, which is a hate crime. If convicted, he faces the possibility of 10 years in prison.

M.B., who is not a Rutgers student, visited Clementi in the dorm room he shared with Ravi on two evenings in the week before his suicide, the last being September 21.

Known only as M.B., the man's identity has been closely guarded because he is considered a victim himself. Efforts to protect M.B.'s identity were requested by his lawyer, Richard Pompelio.

He said in court papers that M.B., who apparently met Clementi online, has an "overwhelming" fear that release of his identity will lead to a "total invasion" of his privacy.

Thus far, the judge has agreed to place tight restrictions on the disclosure of his identity, ordering that M.B.'s name and date of birth be given only to Ravi, his attorney and his attorney's investigator, who are bound not to disclose these details to anyone else.

It was not clear the extent to which his identity will continue to be shielded once he takes the witness stand in the trial in Middlesex County, New Jersey, court.

Students who lived in the same college dorm and have been called as witnesses in the trial described M.B. as about 30 years old and "sketchy."

(Editing By Barbara Goldberg and Paul Thomasch)