I can understand your frustration since regnal naming conventions are arbitrary schemes cooked up in modern times to
help avoid confusing similarly named people. And since they are arbitrary there isn't a lot of
consistency.
Still, for whatever it's worth, in the case of Julian the II *is* appended in recognition of the usurper. It is why we get
Constantine III.
Like it or not this algorithm seems to be the "generally recognized consensus" for the Roman/Byzantine age:
Add numeral to successor if held title of
Augustus where
cognomen was the same as a previous
Augustus except not if ultimate rank attained was only
Caesar unless
his caesarship was
contemporary to an eponymous
Augustus or if the ruler's name was typically styled along with
his nomen or he is better known by a nickname (!)
In this scheme we see why, for example,
1-
Didius Julianus is not referred to as Julian I for the
Didius is always accompanied with the
Julianus2- For having historically catchy nicknames we don't see the arguably more appropriate Antoninus II and III, respectively
3- We refer to
Valerian II and
Licinius II though they were only caesars (to avoid confusion with their fathers)
So in the end whether or not the ruler usurped power or not is irrelevant so long as he was recognized as an emperor in
his home province (and in effect we should recall that a majority of non-dynastic successions started out in one extra-legal fashion or another). In practice, however, we do tend to drop the numerals when the successor was only of trivial historical importance.
Ras
No, with all due respect, historically speaking Julian of Pannonia was NOT "Julian I." He was not an emperor but a usurper, or rather a rebel as he never successfully usurped the throne and only VERY briefly controlled a small part of the east before he and his rebel army entered the Italian penninsula to be promptly slain by Carinus at Verona when they attempted to march on Rome. He was not considered to be a legitimate emperor by the Romans either then nor later, and historians do not regard him as such now. He is in the same class as so many other usurpers and rebels who optimistically struck coinage but who were unsuccessful in their revolts and quickly supressed: important as the issuer of now-rare coinage and locally important in the history of the Roman provinces and the Empire's internal wars, but NEVER an emperor.
The legitimate emperor Julian, grandson of Constantius Chlorus, nephew and son-in-law of Constantine the Great, brother of Constantius Gallus and cousin and brother-in-law of Crispus, Constans, Constantine II, and Constantius II, never styled himself "Julian II" and it is quite safe to say that neither he nor anyone in the Roman Empire in his day considered Julian of Pannonia to have been a legitimate ruler.
