What was the nature of Mileva Marić-Einstein’s relationship with Albert Einstein?

Mileva Marić-Einstein and Albert Einstein met at the Federal Polytechnic School in Zürich, developed a friendship that turned into romance, were married, and had three children. Their marriage ended in divorce, but nearly the entirety of their relationship was chronicled in 54 letters they wrote to each other, which were discovered in the 1980s and later published.