Everyone has heard about Albert Einstein, but Einstein would probably not have become Einstein if not for BFF Marcel Grossmann.
Grossmann, a Jewish Hungarian-born mathematician, helped Einstein numerous times. When the then-unknown genius was struggling with unemployment, Grossmann's father got young Albert a job in the now-famed patent office that kick-started his career. Later, it was Marcel himself who helped Einstein get a job as a physics professor at Zurich Polytechnic (the Fightin' Zebras?).
Perhaps more importantly, Grossmann helped Einstein with that little nugget known as the Theory of Relativity. His knowledge of mathematics complemented Albert's knowledge of physics. The duo collaborated on a paper that became foundational to the theory.
Also, while the two were in school together, Einstein used to cut class a lot. Grossmann attended each lecture so that his friend could pass by using his copious notes...
Now that's true friendship.