"It was very intense, and everything was done in English," said Yuan, whose mother, Patricia Tsien, later started another gigantic project to translate Koo's oral history into Chinese. The result was more than 6 million Chinese characters.

"For 13 years, my mother worked with a group of more than 30 translators based in Beijing and the neighboring Tianjin city. She went to China to meet every one of them, who later wrote constantly to her asking all sort of questions like 'What is this person's Chinese name?' and 'Where was Mr Koo exactly at this time?'," she said.

"With oral history, there can be gaps, gaps that my mother worked very hard for many, many years to fill," said Yuan, a Harvard-educated anthropologist who herself helped with researching and fact-checking.

Along the way, pictures were collected and part of them eventually went to the memorial museum for Koo in the Jiading District of Shanghai, where he was born in 1888.

Tsien passed away in 2015, at age 97, pleased with the fact that her father was able to see the very first of the 13-volume translation before his own passing, also at the age of 97, in 1985.

Both Yuan and Young have become the torch bearers for Koo's legacy over the years.

After Koo's passing, Young, together with her mother, donated to the memorial museum cases of books from his bedroom, books in which Koo "circled things".

"Koo kept everything, including a thank-you note I wrote him at age 11, after receiving a box of chocolates he sent to me. All those records were given to Columbia and put into their Rare Book & Manuscript Library," she said.

"Four years ago, I brought Columbia and a number of Chinese institutions together, in an effort to digitalize the materials and make them available to researchers in China," said Young, who described Koo's marriage with her mother as "one of great affection and devotion" and Koo himself as "fun-loving".

"Every night, my mother would put a glass of Ovaltine, which is a chocolate milk drink, outside his room, in the hallway. And then the next morning, she would check to make sure that he had drunk it－he would usually get up at around midnight after he went to bed."

V.K. Wellington Koo died peacefully on Nov 14, 1985, and was buried at the Ferncliff Cemetery and Mausoleum in Hartsdale, New York. There was a diary entry that day in the morning. It says, "A quiet day."

Juliana Young Koo, as she signed her name on the cover of her autobiography 109 Springtimes－My Story, published in 2015, passed away in 2017, at the age of 111.

Oei Hui-lan, known by her friends as Madam Koo, died in 1992 at the age of 100. Her autobiography, published in the 70s, is titled No Feast Lasts Forever.

On June 28, 1919, the very last day of the Paris Peace Conference, Koo, having decided not to turn up and sign, "drove slowly in the morning twilight".

"Everything looked so sad to me－the color of the sky, the shade of the trees and the deserted streets. I thought the day must remain in the history of China as the day of sorrow," he wrote in his diary.

Young, who organized a memorial concert for Koo at New York's Carnegie Hall last year, understands all that.

"If he died in his 60s or 70s, he might have felt a sense of failure. But he lived a long life, which allowed him to take great comfort in the fact that China was finally on its way to become a major world power. Koo was a true globalist with a distinct vision for China in the global community," said Young, who in 1989 co-founded the Committee of 100 with such prominent Chinese Americans as the late architect I.M. Pei and master cellist Yo-Yo Ma. The committee, whose members are "American citizens of Chinese heritage", is dedicated to advancing dialogues between the US and China.

Appearing at the memorial concert in New York, Chinese Consul General Huang Ping called Koo's achievements "difficult for us to reach". "These books (his 13-volume oral history) are must-reads for Chinese diplomats," he said.

Back in the late 1890s, Koo was passing a bridge over the Huangpu River in Shanghai when he saw a plump Englishman whipped a rickshaw puller, despite all the difficulty the Chinese was having pulling the cart up slope against the wind.

At a time and a place where, to use Young's words, "foreigners were king", the teenage boy stood in the wind on the sidewalk and shouted to the man what must be the biggest insult he could possibly thought about－"Are you a gentleman?"

"It was too much for a boy of that age to understand political reforms, but I could feel that something was wrong and needed to be corrected," wrote Koo, who years later would argue on a world stage for China to take back the concessions and abolish all unequal treaties. "I made up my mind to be a diplomat."