Einstein's close friend and collaborator at the Swiss Patent Office was Michele Besso. Besso was a Swiss-Italian engineer who became Einstein's colleague at the Patent Office in Bern after Einstein helped him obtain the position.
The two men had first studied in Zurich and remained lifelong friends. At the Patent Office, they discussed physics alongside other members of the informal Olympia Academy. Einstein valued Besso as an unusually perceptive sounding board and later called him “the best sounding board in Europe.”
Besso is especially associated with Einstein's 1905 work on special relativity. In the paper introducing the theory, Einstein explicitly thanked his friend and colleague for valuable suggestions. Besso also discussed problems in general relativity with Einstein, including the precession of Mercury's orbit.
A common mix-up is Marcel Grossmann, another important Einstein collaborator who helped with the mathematical structure of general relativity. Grossmann was not the close Patent Office colleague identified by this question. Besso and Einstein remained in contact until Besso's death in 1955, just over a month before Einstein died.