Rabbi Ephraim Eliezer HaKohen Yolles (also spelled Yales; born 5 Shevat 5651 (1891) – passed away 25 Cheshvan 5749 (1988)) was the Rav of Sambor and later the Chief Rabbi of the Orthodox community in Philadelphia.
In the printed Igros Kodesh he is usually addressed by his initials — מוהרא"א הכהן or מהורא"א הכהן (Moreinu HaRav A. A. HaKohen) — and the editors' note identifies him as "יאלעס". All the letters so addressed are collected here.
After the arrival of the Previous Rebbe (Rabbi Yosef Yitzchak Schneersohn) in the United States, Rabbi Yolles approached him with a request to study Kabbalah and Chassidus together. The Previous Rebbe referred him to his son-in-law, Rabbi Menachem Mendel Schneerson (the Rebbe). After the passing of the Previous Rebbe in 1950, Rabbi Yolles was among those who strongly encouraged the Rebbe to accept the leadership of Chabad.
From then on, Rabbi Yolles became a regular participant in the Rebbe's farbrengens and formed a deep personal connection with him. He regarded the Rebbe as his personal spiritual guide and would visit him several times each year.
Their closeness is evident in the Igros Kodesh, which contains a large body of Torah correspondence between the Rebbe and Rabbi Yolles, from the early 1940s onward, in both Nigleh and Kabbalah.
Rabbi Yolles would bring even complex Torah-and-science questions to the Rebbe. A well-known example concerned the size of the sun as given in the Rambam and Tanya against the measurements of modern astronomy; the Rebbe's reply is in Igros Kodesh vol. 22, letter 8454.
Rabbi Yolles' relationship with the Rebbe was marked by great respect and bittul (self-nullification). He relied on the Rebbe's guidance in Torah study and in personal matters, and remained closely connected to him throughout his life.