Johannes Kepler attended Charles University in Prague. (PROSE: The Long Step Backward) According to the Seventh Doctor, he possessed a "scientific genius". (PROSE: Fable Fusion) The Thirteenth Doctor called him a "mathematical genius" and one of the prime movers of astronomy. (COMIC: Herald of Madness)
He had a wife and two children, Heinrich and Susanna. When they died in infancy in the 16th century, he and his wife moved to Bohemia to start a new life in the new century. By 1601 he was a student of Tycho Brahe, who showed him and his wife kindness.
While accompanying Brahe to a gathering in Castle Houska, he met the Thirteenth Doctor, Graham O'Brien, Ryan Sinclair and Yasmin Khan in Kokorin Forest in Central Bohemia. His appearance greatly impressed the Doctor, although her group quickly realised he was still just Brahe's apprentice at this time. While at the gathering, Yaz started talking to Kepler, where he explained his past to her. After the Doctor noticed that something was wrong about the meeting, Yaz promised to stick with him, but she lost him after Baroness Dagmar Ruskovitch invited him to see her collection.
After being dragged away by the Baroness, Kepler was led into a massive library, where Ruskovitch promised to share with him "inspiring" new knowledge via a glowing orb. However, Graham, Yaz, and Brahe interrupted the proceedings to save him from obtaining knowledge from the future. When she attempted to kill them, Kepler stopped her, letting her use him on the basis that she let the others go free. After Ruskovitch paralysed the assembled astronomers with the orbs she had given him, she ordered Kepler to finish her plan, just as a huge hand appeared out of the castle's lower chamber. With Graham and Yaz breaking the orbs and freeing the astronomers, the Doctor's group went to find her again, and Yaz led Kepler to safety as Ruskovitch revealed to the Doctor that she did not summon the entity, known as the Herald; she was, in fact, attempting to prevent it from appearing - something that Kepler alone was aware of.
As the Doctor and Ruskovitch hunted the Herald, Kepler prepared to seal it away at the cost of his life, but Brahe and Graham protested, knowing of his impact in the future. With Kepler unconscious, Brahe took his place, dying in the process. The next morning, Kepler tried to help Graham by insisting that he would be given a state funeral and would always be remembered a hero. (COMIC: Herald of Madness)