Sigma represents Boyfriend's greed for Fame and Recognition
Theory posited by a comment under this video
.As you may know, this game has a couple of references to The Binding of Isaac. Primaraly Sigma and his more the passing resemblance to Delirium. Since it and most of it fellow final bosses tend to be symbolic in some way (Delirum itself being the "white light at the end of the tunnel" and My Life Flashed Before My Eyes, Dogma representing the Christian broadcasting Isaac's mom watches, and the best ultimately representing Issac's various traumas), it wouldn't be too much of a stretch that Sigma represents something similar.In this case, it represents Boyfriend caring less about rapping for her girl and more for the greed of Fame and recognition. The fact that the entire gameplay of FNF revolved around copying your opponents, and Sigma himself shapeshift into them (primarily ones already in are planned to be in the main game), strengthens this idea. It would also explain the random BF Embryo in Sentimental to an extent.
