Who Was Rav Aharon Kotler's 'Rebbe'?

Rabbi Shneur Kotler related that when his father, Rabbi Aharon Kotler, was hospitalized for medical treatment, Reb Aharon asked if anyone had mentioned his situation to 'the Rebbe'. "To which Rebbe is the Rosh Yeshiva referring?" asked one of his relatives. "When I say 'the Rebbe,"' Reb Aharon retorted, "I mean the Kopycznitzer Rebbe." Reb Shneur added that his father sent every chassan in the yeshiva to the Kopycznitzer Rebbe to receive a beracha before the wedding. At a certain gathering of gedolei Torah at which both Reb Aharon and the Kopycznitzer Rebbe were present, Reb Aharon turned to the Rebbe and asked his opinion on the issue under discussion. When Reb Aharon was later asked why the Rebbe's opinion was so important to him, he answered, "The Torah was given to the Jewish People in order to purify them - and the Kopycznitzer Rebbe has purified himself' (Heard from the Rebbe's grandson, Rabbi Avraham Yehoshua Heschel).