Her mix of hidden virtuosity and tender emotion complemented the power of John Kriza, her frequent ballet partner, as Billy. Both performed the ballet with the company before President John F. Kennedy and the first lady, Jacqueline Kennedy, at the White House on May 22, 1962.
Ruth Ann Koesun was born on May 15, 1928, in Chicago to Dr. Paul Z. Koesun, a prominent Chinese physician in Chicago’s Chinatown, and the former Mary Mondulick, who was of Russian descent.
She met Mr. Kriza, a Ballet Theater star, while studying at the Chicago ballet school directed by Bentley Stone and Walter Camryn and performed with him in nightclubs and summer stock.
Ms. Koesun joined the company in 1946 while it was appearing in London and advanced quickly to principal dancer. She retired from Ballet Theater in 1969. Her marriage to Eric Braun, a principal dancer with the troupe, ended in divorce after eight years.
Ms. Koesun, who lived in Chicago, leaves no immediate survivors.
Her first years in the company were auspicious. The cover of the Nov. 3, 1947, issue of Life magazine consisted of a double portrait of two young Ballet Theater dancers, Ms. Koesun and Melissa Hayden.
Rarely had a novice received such rave reviews as those that greeted Ms. Koesun in May of that year for her role in Ballet Theater’s “On Stage!” Michael Kidd, the future Broadway and Hollywood choreographer, had cast her as a very young dancer suffering from stage fright at a rehearsal.
“It was little Ruth Ann Koesun who captured all hearts last night,” the critic John Martin wrote in The New York Times. He praised her “lovely line” and “immeasurable charm.”
Walter Terry, the dance critic for The New York Herald Tribune, wrote later that year that Ms. Koesun’s progress “has been so rapid” and hailed “her highly individual quality of movement, shy but not weak, delicate but assured.”
“She had a unique style,” Richard Covello, an arts writer in Chicago who followed Ms. Koesun’s career closely, said in a telephone interview, noting her hidden technical strengths. “She was a strong dancer locked in the dainty,” he said.
In 1951, Mr. Martin wrote of her “exquisite dancing” in the “Bluebird” pas de deux with the French guest star Jean Babilée, adding that he wondered why “a dancer of these potentialities does not always exhibit them.”
Over the years, Ms. Koesun’s other partners included Erik Bruhn, and her repertoire included Mr. Tudor’s “Dim Lustre,” David Lichine’s “Graduation Ball,” Bronislava Nijinska’s “La Fille Mal Gardée,” Frederick Ashton’s “Les Patineurs” and new works as well, among them Mr. Babilée’s “Till Eulenspiegel.”
She performed often in Jerome Robbins’s early ballets, including “Fancy Free” and “Interplay.” Mr. Covello said that she was the only dancer to perform each of the three female roles in “Fancy Free.” This was in part thanks to a fluke.
The ballet ends with three sailors chasing a young woman who appears very briefly onstage after the two main women in the ballet have left. In previous performances, Ms. Koesun had danced each of the two main female roles. On this occasion, she was in the wings when the third woman was late and did not appear.
“So,” Mr. Covello said, “Ruth Ann went back onstage and did the part.”