We know that he exists, and
might be a very young man—a particular
side story mentions that Titus has no ready heir circa Kaien's rebellion. Nerva is Titus's son and thus Zenos's uncle, which makes this hard but not impossible to reconcile with him being the Legatus of the IIIrd Legion.
We also know that he is supposedly being backed for the throne by House Brutus. Finally, we know that Lyon, one of the top men of the IVth Legion, believes him to be "weak and a coward", and supports Gabranth's decision to abandon Garlemald instead of following Nerva's demand that they return to the capitol.