All Spoilers Are Unmarked. You Have Been Warned
Lemmings is Idol Singer Ryouma Kannagi.
Hints have been piling up that this is the case, such as Lemmings and Ryouma being out of town at the same time, Lemmings having a main job that he has to prioritize over working for Takeru and him having to hide his face at all despite almost no one else in the underworld doing so, implying he's doing it because he could be recognized. Other things such as the two sharing a voice actor in the anime and his schedule showing TV appearances all but confirm it.