Short answers, pulled from the story.
Dharmakirti was an influential Indian Buddhist philosopher who worked at Nalanda, the great monastic university of Magadha. Most scholars place him between 600 and 660 CE, with the Chinese monk Yijing describing him as a "recent" teacher during Yijing's residence at Nalanda from 675 to 685 CE.
Dharmakirti's Pramanavartika is his largest and most important work. It is a commentary on Dignaga's Pramanasamuccaya and became a central text on pramana, the theory of valid knowledge instruments, in both India and Tibet. It was widely commented on by Indian and Tibetan scholars and remains part of the basic monastic curriculum in Tibetan Buddhist monasteries.
Dharmakirti held that there are exactly two instruments of valid cognition: perception (pratyaksa), which is a non-conceptual knowing of particulars bound by causality, and inference (anumana), which is conceptual, linguistic, and reasonable. He defined a pramana as a "reliable cognition" whose reliability consists in its compliance with an object's capacity to perform a function.
Tibetan hagiographies describe Dharmakirti as having been born into a Brahmin family in South India and as being the nephew of the Mimamsa scholar Kumarila Bhatta. When Dharmakirti was young, Kumarila spoke abusively to him as he was taking his Brahminical garments, which led Dharmakirti to take the robes of the Buddhist order instead, resolving to "vanquish all the heretics."
Indian and Tibetan doxographers have long disagreed on how to categorize Dharmakirti. The Gelug school asserts he expressed Yogacara views, most non-Gelug Tibetan commentators assert he expressed Sautrantika views, and some later Indian Madhyamikas reportedly claimed he expressed Madhyamaka views. Modern scholars including Tillemans, Amar Singh, Christine Mullikin Keyt, and Dan Arnold continue to debate this, with Arnold arguing that Dharmakirti's alternating perspectives are compatible and applied at different levels of analysis.
Dharmakirti's theories became normative in Tibet, where his texts form part of the basic monastic curriculum to this day. Phya pa Chos kyi Seng ge (1182-1251) wrote the first Tibetan summary of his seven treatises, reading Dharmakirti as a realist. Sakya Pandita then wrote a competing interpretation treating him as an anti-realist. These two rival readings became the foundation for most subsequent debates in Tibetan epistemology.