First Appearance: Tomb of Dracula #51 (December 1976).
Appearance: Tomb of Dracula #51, 52, 54, 55, 61-63, 65, 67, 68, 70.
Years Active: Centuries ago to the present.
Janus was an angel who opposed Dracula at various points in time. Janus has been active for centuries, although exact dates for his existence are not available. Before the merging he was only an angel; this angel was never given a name, although the Official Handbook of the Marvel Universe refers to him as the "Golden Angel." ("Janus" was his later designation, and I'm going to use it, as I find it more euphonious.)
Janus' first encounter with Dracula took place some centuries ago, when Dracula was ruling a small province in Spain and planning to take over Spain itself. Dracula was about to execute a spy when the angel prevented the killing with only a word; Dracula was unnerved enough by this encounter that he fled Spain that night. In 1870 Dracula led a vampire horde against the rulers of Vienna; Dracula found Janus waiting for him in the city's throne room and fled.
In modern times Janus revealed that he "returned home" when his job--opposing Dracula--was done. In the modern era he merged himself with with Dracula's infant son, who was born to the "pure-hearted Satanist" Domini. In this new form he had various adventures and clashes with Dracula before ascending into Heaven.
Janus' powers include the ability to project beams of concussive force from his eyes and hands, radiate blinding light, freeze people within 'a fraction of time', cause Dracula pain just by looking at him, flight, weather control, and immunity to vampiric hypnotism and the ability to break others from it.
Note: There are at least two other "Janus" characters in Marvel continuity. I'm assuming this Janus was the first and so am going to label him as such.