One was among the most brilliant composers and conductors of the 20th century, a sexagenarian who brought the world one of the great love stories of musical theatre. The other was a 26-year-old who worked in a Tokyo insurance company. Now, a newly revealed cache of letters shows that in the last decade of his life Leonard Bernstein embarked on a passionate relationship with a Japanese man.

“I noticed that he was gazing upon me,” Kunihiko Hashimoto, now 66, said. It is hard to explain about his eyes. It was not to try to talk to me nor seduce me, just he was looking at me. It was irresistible.”

Hashimoto’s role in Bernstein’s life had been almost completely overlooked until Mari Yoshihara, a Japanese academic, discovered some 350 letters at the Library of Congress in Washington. She found that they first met in 1979, when Hashimoto went backstage with a friend after a concert and ended up spending the night with Bernstein.

The West Side Story composer was visiting Tokyo with the New York Philharmonic and when he left Hashimoto was devastated. The following day, he wrote to him: “After you left Japan, my mind became vacant, because the one night and afternoon that we had were like a beautiful dream.”

Yoshihara, professor of American Studies at the University of Hawaii, said her “jaw dropped” when she first read the letters, having stumbled across them by chance in the library’s Bernstein collection.

“They’ve got 1,700 boxes of some 400,000 items,” she said, “including manuscripts… It’s completely overwhelming. I was going through it and, by accident, came across these personal letters to Bernstein from two Japanese individuals whose names I didn’t recognise.”

Intrigued, she asked to see the rest of the correspondence: “It completely changed the course of my project.”

Hashimoto’s letters span about 11 years until the summer of 1990, a few months before Bernstein’s death, aged 72. Yoshihara found that the Japanese man’s name appears only briefly in the most comprehensive Bernstein biography – as a business representative.

“Beyond that, I couldn’t find anything about him,” she said. “He was working for an insurance company when he first met Bernstein. A few years after their relationship began, because Hashimoto was already an amateur actor, he auditioned for a major musical theatre company in Japan and became an actor. Bernstein’s manager later invited him to become the composer’s Japan representative ... Hashimoto played a crucial role in some of Bernstein’s major projects in his late career, including organising the Hiroshima peace concerts in 1985.”

Their relationship went far beyond sexual intimacy. Hashimoto went on to work as a writer, director and producer in Australia and Japan, and his productions included a translation of Bernstein’s musical Candide.

Bernstein, according to his West Side Story collaborator Arthur Laurents, was “a gay man who got married. He wasn’t conflicted about it at all. He was just gay.”

After the death of his wife, actress Felicia Cohn Montealegre, in 1978, he had many lovers. Although Hashimoto was not the only object of Bernstein’s affections, the love was not unrequited, according to Yoshihara: “It was mutual. On two occasions Bernstein arranged to bring Hashimoto to Europe to spend time with him.”

In one letter, Hashimoto wrote: “I have received the tenderest letter from you. I read it over and over again.”

Describing the correspondence as “passionate, tender and sometimes heartbreaking”, Yoshihara said: “Hashimoto’s words exude every emotion of a young man who has fallen deeply in love.”

The Japanese man was taken aback when he was told by Yoshihara that she had read his letters. “He was very shocked … He hadn’t even known that the letters were available for viewing at the Library of Congress,” she said. But she secured his trust once he realised that her forthcoming book, Dearest Lenny: Letters from Japan and the Making of the World Maestro (Oxford University Press, £19.99), is a scholarly study of Bernstein interwoven with the history, politics and economics of the 20th century.

It will include Hashimoto’s letters, one of which reads: “I never forget that you asked me where we should live. Your question was where ‘we’ should live, wasn’t it?’ I would like to live with you. Even as a maid (but I am not good at cooking and sewing). Even as a secretary (but I cannot type quickly). Even as like a doll (but I have a mind) … I was born to meet you and to be with you.”

Elsewhere, he wrote: “The letters you wrote to me … the pictures I took in Munich, Positano, Amsterdam, and the presents you gave me … I have kept all of them and locked them in a briefcase.”

Hashimoto said last week: “I sealed Lenny’s … memories in a suitcase – not only his letters. Because if I open it, all our memories will burst out. It [makes] me so emotional.”

Yoshihara’s book, which is to be published on 2 September in the US and 28 November in the UK, also includes previously unpublished missives from a Japanese woman, also held in the Library of Congress: Kazuko Amano began writing fan letters to Bernstein in 1947 and became a close family friend over more than four decades.