Ethan Amberglass was a mathematician.
He helped the Seventh Doctor stop a conceptual alien from another dimension from invading our universe. Ethan was immune to their influence due to having a brain tumour. He and Ace had a sexual relationship for several years, then he slipped into a coma. The Doctor returned and made a mindscape of the Riemann hypothesis for him to spend his last year of life exploring. (PROSE: The Algebra of Ice)