Googling for "persona", I found this definition of persona in Roman Satire in this JSTOR (limited access) article/review:
Review: Themes in Roman Satire
Author(s) of Review: S. H. Braund
Reviewed Work(s): Themes in Roman Satire by Niall Rudd
The Classical Review > New Ser., Vol. 37, No. 2 (1987), pp. 207-209
A fundamental aspect of the study of Roman verse satire [is] the concept of the persona. By persona I mean the mouthpiece created by the poet, whose voice is the voice we hear in the satires; sometimes we are invited to identify the persona with the poet, as for example in the case of Horace; sometimes the persona is given a different identity. either named (e.g. Juvenal's Umbricius in Satire 3) or left anonymous. The chief significance of the persona concept is that it frees us from the biographical fallacy of relating the satires - to the poet's own life and experiences on a crude. literal basis (for exposition of the persona approach to satire. see W. S. Anderson, Essays on Roman Satire. Princeton. 1982)
Further search led me to a Bryn Mawr Review of The Roman Satirists and Their Masks. Bristol: Bristol Classical Press, 1996 by Susanna Morton Braund (same person as reviewer above?). More on the concept and history of persona can be read in this review.