a freedman of Caligula, in whose reign he possessed very great influence and power, though in the end he was an accomplice in the conspiracy by which this emperor was murdered. In the reign of Claudius, Callistus continued to have great influence, and endeavoured secretly, in conjunction with others, to counteract the attachment of Messalina to C. Silius; but Callistus was afraid of losing his position, and gave up opposing the scheme of Messalina. When she had been put to death, Callistus supported the designs of Lollia Paulina, who wished to become the emperor's wife; but he did not succeed in this point, for Claudius married Agrippina, who was supported by Pallas. This Callistus is undoubtedly the person to whom the physician Scribonius Largus dedicates his work; and from it we learn that the full name of Callistus was C. Julius Callistus. (Tac. Ann. 11.29, 38, 12.1, &c.; D. C. 59.19; Senec. Epist. 47; J. AJ 19.1.10.)
[L.S]A Dictionary of Greek and Roman biography and mythology
Smith, William
A Dictionary of Greek and Roman biography and mythology. William Smith, LLD, ed. 1890