Lem's mother, Millicent, is a direct descendant of Koichi, a Sorreter and adventurer who moved to First Village in 399. Since that time, it has been a common practice for each generation of Koichi's descendants to teach the next magic. However, it's rare for any of them to be certified as adepts by the Sorret Magic Academy or the Council of Magicks, as most of Koichi's descendants apparently never visit Sorret. Millicent herself was never certified, though it is believed she was at master-adept level (which is presumably the case for all of Koichi's descendants). She taught her son, Lem, magic from an early age, and in 905, Lem translocated to Sorret, where he was tested by the Council of Magicks, and granted master-adept status. Thereafter, he returned to First Village and joined the Black Profits, serving under its Chief Sorreter, Gillian Mancer.
When the surname law was passed in 904, Lem's family chose the name 'Delphi,' though they've never publicly explained why.
Lem quickly proved himself to be the most talented Sorreter in Mancer's department, which earned him unofficial 'second in command' status. There were other Sorreters who'd been in the department longer than he, but apparently none of them resented his swift rise. Partly this is because they recognized his talent, and partly because they knew and respected his family, who had close ties with other important families within the BP, including that of don Dirk Noir XIX. And even before officially joining the BP, Lem had spent a great deal of time associating with members of its various departments, especially the sorretry department. So it came as no surprise when, shortly after Mancer's death in Su'yet of 912, Noir appointed Lem to the position of Chief Sorreter.