Dr. Malcolm Polstead was a scholar at Jordan College at the University of Oxford in Brytain who watched over Lyra Silvertongue from the time she was a baby. His dæmon was Asta, who settled as a ginger cat.
As a child, Malcolm lived with his parents at the Trout Inn near Oxford, and helped at the tavern to serve patrons there. He went to Ulvercote Elementary School, in addition to working as a jack-of-all-trades for the nuns at the Godstow Priory across the river. In his spare time, he enjoyed playing with his dæmon in his canoe, La Belle Sauvage.
When he was 11, Malcolm befriended the alethiometrist Hannah Relf after recovering one of the carved acorns used the organisation she worked for, Oakley Street. She recruited him as a casual informant on all the unusual goings-on happening in Oxford, privy to a wealth of information as a result of his time in the crowded inn and his relationship with the nuns. Given the lack of free libraries in Oxford and Ulvercote's status as a trade school, she let him borrow from her own collection. 
Malcolm started his watch over Lyra when she was first brought to the priory as an infant, to be taken care of by the nuns at Lord Asriel's request. Enchanted with her, he grew to be very protective, wary at any hint of a threat. Malcolm escaped with her and Alice, a sixteen-year-old girl who worked part-time at the Trout, in La Belle Sauvage as the Flood of 1986 caused the river's banks to burst and devastate the priory, in addition to being pursued by Gerard Bonneville, a man intent on taking Lyra. Over the course of the journey to Lord Asriel's home in Chelsea, Malcolm managed to take a rucksack containing a stolen alethiometer and several folders' worth of research from Bonneville during one of their encounters, and finally killed the man with the paddle of his canoe. On Malcolm's advice, Lord Asriel took all three children back to Oxford, to invoke the right of scholastic sanctuary from the Master of Jordan College. Malcolm placed the alethiometer in Lyra's blankets for its protection and as a gift.
He went on to obtain a doctorate in the next decade and worked out of Jordan College as an unusually young scholar. Malcolm taught Lyra when she was younger for a brief six weeks, and later became someone she was comfortable enough with to approach with questions in a letter about her dissertation.