That year, after being injured excavating a sample of a dark matter asteroid, Tilly began to see a hallucination of Ahearn as a Starfleet engineering officer aboard the USS Discovery. Thinking she was an actual member of the crew, Tilly used her as a sounding board to work out how to save the planet of Terralysium from radioactive debris. The hallucination of Ahearn encouraged her to use the asteroid itself to collect the debris, and later appeared on the bridge to congratulate her.
Tilly was able to identify her by the use of Tilly's old nickname of Stilly, and attempted to locate her on the starship. When advised by the ship's computer that Ahearn was not on board, Tilly called up Ahearn's file, and learned of her death. (DIS: "New Eden")
As time went on, "May" became more and more insistent, making references to a "plan" that she needed Tilly to enact. After an outburst at "May" on the Discovery's bridge, in full view of Captain Christopher Pike and the senior staff, Tilly admitted to her friend Michael Burnham that she had started seeing a "ghost" after the incident with the asteroid, and suspected she was going insane. However, when Tilly mentioned that "May" seemed confused at the concept of tears, Burnham realized that "May" was much more than a hallucination, and deduced that the dark matter had reacted to exposure to the mycelium spores around which Tilly regularly worked. Astromycologist Paul Stamets, whom "May" believed was in fact the captain due to his being the interface that powered Discovery's spore drive, confirmed that a multidimensional fungal parasite had grown inside Tilly's body, grown from a spore native to the mirror universe which had attached itself to Tilly during their escape from that dimension; First Officer Saru speculated that it had taken the form of May through brain manipulation. Stamets used a piece of the asteroid Tilly had obtained using a laser core sampler to remove "May" from Tilly's body. (DIS: "Point of Light")
The form of May Ahearn was played by actress Bahia Watson.