This summary is drawn from the Rebbe's letters to him (about 5703–5724, 1943–1964); I did not verify further biographical details from published sources.
A shadar in Europe. The early letters show Rabbi Shem-Tov serving as a shadar (emissary) of the Chabad institutions, including in Paris. The Rebbe encouraged him that the mission is G-d's work, to use his time wisely and to serve with joy (IK 446), and asked him to share Torah teachings with others (IK 631).
Steadfast in the face of criticism. When he faced criticism of Lubavitch activities, the Rebbe wrote that opposition to holy work is nothing new, and that steadfast dedication to the mission ultimately prevails (IK 2401). He encouraged organizing Shabbos gatherings, which strengthen communal bonds and personal growth alike (IK 2422), and taught that real progress comes through concrete action in the present, not only hope for a better future (IK 4029).
Chassidic avodah. On the avodah of Erev Yom Kippur, the Rebbe wrote that even when severity is needed in serving G-d, it should always be covered over with kindness (IK 299).
Distributing sichos. On establishing an office to distribute the Rebbe's sichos, he stressed that only properly edited sichos should be distributed, to avoid errors being attributed to him (IK 9309).
Sources: the Rebbe's letters cited above.