Okay, so I've given some thought to this, and I believe that Grisha Yeager and Zeke the Beast Titan were either relatives or friends before the fall of the Walls. I'm thinking something like, they worked together on the program to create the Titans, but when Grisha realized their purpose, he rebelled, so Zeke turned him into a regular Titan and turned him out into the world. But many years later, Grisha ate a shifter and regained his humanity, but his abilities came back slowly over the 20 years of his life within the Walls.
This would explain his apparent knowledge of the Titans and the Coordinate, as well as his determination to stop them.
What do you guys think?