The Hebrew name כסה appears to be Yonah Kesse, a Labor (Mapai) public figure and Knesset member in Israel; I did not verify this identification from a published source. The summary below is drawn from the Rebbe's letters to him (about 5723–5728, 1963–1968).
Public service and Kfar Chabad. The Rebbe encouraged continued dedication to public service and support for Kfar Chabad (IK 9505), taught that supporting Torah institutions brings satisfaction Above and below (IK 8706), and urged redirecting political passion toward constructive service of the community (IK 9802).
Memories of the Rebbe's brother. The Rebbe expressed deep appreciation for memories Kesse shared about the Rebbe's brother (IK 9185).
Anniversaries. Anniversaries should inspire renewed dedication to Torah values (IK 9835).
Sources: the Rebbe's letters cited above.