More or less as Akuser said, Lily was the reward for 3-medalling map 1-2 of the event. Once you got her, you had to farm chests for special skill and dress blooms for her. Those things most commonly dropped on event maps 1-4 and 2-4 (also known as ?-4 and ?-8 ), so you had to run those two maps 3 to 4 times each to get all the blooms to unlock all her skill levels and equipment slots.