Short answers, pulled from the story.
Marcel Grossmann was a Swiss mathematician born on the 9th of April, 1878, who was a close friend and classmate of Albert Einstein. He introduced Einstein to Riemannian geometry and tensor calculus, the mathematical tools that made the general theory of relativity possible, and co-authored the foundational 1913 paper Outline of a Generalized Theory of Relativity and of a Theory of Gravitation.
Grossmann taught Einstein the absolute differential calculus developed by Elwin Bruno Christoffel, Gregorio Ricci-Curbastro, and Tullio Levi-Civita, and emphasised the importance of Riemannian geometry as a framework for describing gravity. He also mentored Einstein in tensor theory, according to Abraham Pais's book on Einstein.
Grossmann and Einstein published Outline of a Generalized Theory of Relativity and of a Theory of Gravitation in 1913. It is considered one of two papers that established the foundation of Einstein's theory of gravity.
Grossmann graduated from the Federal Polytechnic School in Zurich in 1900, earned his doctorate from the University of Zurich in 1902, and became a full professor of descriptive geometry at the Federal Polytechnic School in 1907. He was also one of the founders of the Swiss Mathematical Society in 1910 and an invited speaker at the International Congress of Mathematicians in 1912 and 1920.
The Marcel Grossmann Meetings are international gatherings held every three years by the relativistic physics community to honour Grossmann's contributions. The Marcel Grossmann Awards, administered by the International Center for Relativistic Astrophysics, recognise outstanding work in the field and each recipient receives a silver casting of the T. E. S. T. sculpture by artist A. Pierelli. Past individual recipients include Stephen Hawking, Shing-Tung Yau, and Tsung-Dao Lee.
Grossmann's father helped Einstein obtain his position at the Swiss Patent Office in Bern. Grossmann himself later helped negotiate Einstein's return from Prague to become a professor of physics at the Zurich Polytechnic. During their student days, Grossmann shared his detailed lecture notes with Einstein, who frequently missed classes.