Jacob is a mysterious child of German background who had some unspecified connection to Rolf Müller, and perhaps Hooded Justice. Judging by his appearances, he must have been born around 1910 or later.
Jacob is seen in a picture with Müller suggesting that they were relatives, or that Jacob was Müller's admirer. The picture was in a copy of A Child's Garden Verses dedicated to Jacob. The Comedian finds the book years later in Müller's possession, and is unclear if Müller had given it to him while they were together. In any case, the Comedian speculated that the boy was one of his victims.
Jacob is first seen in a flashback scene from 1920, walking around Hauptzelt Zircus and then meets a menacing strong figure, which obviously represents Rolf Müller, and suggests that, whatever their relationship, Jacob was somehow abused by him.
H.J. could indeed be Jacob, as both have German background. If this is the case, it seems that H.J. was abused physically and sexually by Müller, before leaving him and becoming a masked adventurer. It can be suggested that his traumas where the origin of his sadistic attitude while fighting crime, hinted to several times in the books, and his vicious and homoerotic relationship with Nelson Gardner.