Hieronimo is the Knight-Marshal of Spain and the protagonist of the play. The Knight-Marshal was, in the Spanish government, the top judge responsible for any legal matters concerning the King or his estate. Hieronimo's occupation thus links him to the play's key theme, that of justice and revenge. Hieronimo equates the two frequently, and, indeed, the play seems to support his equation with its various calls for revenge and retribution. Only one character in the play, Alexandro, shows mercy on someone who has wronged him, and in that case the wrong did not end in death.
There are problems, however, with revenge, problems that Hieronimo must face. That Hieronimo does face these problems is what gives him the psychological complexity and verisimilitude typically associated with the tragic protagonist, making Hieronimo a sort of proto-tragic protagonist in English literature. Not even an important character until the murder of his son Horatio, Hieronimo is suddenly thrust into the center of the action. His character then develops over a series of soliloquies, wrestling with several key questions. These questions include: whether to end his misery by suicide instead of waiting to seek revenge, where to seek revenge against murderers with far more influence over the king than he, how to reconcile his duties as a judge with his inability to find justice for his son, whether to leave revenge to God once his legal means are exhausted, and—having decided to seek his revenge—how to do it in the face of enemies who could easily destroy him with their vastly greater influence and power at court.
Hieronimo resolves each of these questions and decides to seek revenge in a Machiavellian, deceitful manner. This is a radical shift for Hieronimo, who effectively adopts the tactics of the murderer Lorenzo against Lorenzo himself. And though his revenge is successful, Hieronimo's grief is not relieved, only death and silence manages to do this.
Hieronimo's conversion to Machiavellianism and his violent, bloody revenge, may raise problems for both an Elizabethan and a modern audience. Sympathizing with someone who reveals himself to be both deceitful and bloodthirsty is difficult. But Kyd does sow the seeds of Hieronimo's conversion in the first Act, when Hieronimo presents a masque to entertain the court. If we think of Hieronimo as an author of stories related to the downfall of Spanish and Portuguese princes (the subject of the masque), instead of a deceiver, then we see that Kyd has foreshadowed Hieronimo's later transformation. And we may see Hieronimo's revenge less as a violent, evil act than as a creative way to find justice in an unjust society.
Bel-Imperia is the main female character of the story, and she has the misfortune to fall in love with both Andrea and Horatio shortly before they die. She also has the misfortune to have an evil brother in Lorenzo and to be the object of Balthazar's affection, when Balthazar is the very man who murdered her beloved Andrea and then went on to murder her beloved Horatio. She is then forced by both her father, the Duke of Castile, and her uncle, the King of Spain—the two most powerful men in the country—to wed this very same Balthazar.
Bel-Imperia does not, however, appear as a victim in all of this misfortune, which is a testament to the strength with which Kyd has portrayed her. He does this by giving her opportunity to display her rhetorical ability in stichomythia (line-by-line exchanges) between her, Balthazar, and Lorenzo. She also has several soliloquies, during which we have access to a mind, an interiority, with very strong opinions, desires, and motivations. We also have evidence that she has the necessary strength of will to act on her desires and motivations; the clearest example of this may be her participation in Hieronimo's revenge playlet, Soliman and Perseda.
Further, we can object that Bel-Imperia may be too calculating, too cold, and that her thoughts focus too much on revenge. Bel-Imperia, indeed, bears a vindictiveness for the wrongs done to her. Her love for Horatio seems partly motivated by a desire to revenge herself on Balthazar (which, of course, disastrously backfires in Horatio's murder). And she spurs Hieronimo on to revenge when he seems to be lazy in pursuing it. But this makes her murder at the end of the play acceptable—more acceptable, perhaps, than Hieronimo's actions. And as the litany of misfortunes above indicates, if she is angry, then she has very good reason to be.
Lorenzo is an example of the Machiavellian villain, typical of many Elizabethan tragedies and dramas. Other good examples of this type of villain include Iago, from Shakespeare's Othello, and Marlowe's Barabas from The Jew of Malta. This character exploited the popular disapproval of the early sixteenth-century Italian political philosopher Niccolo Machiavelli, whose The Prince portrayed a picture of a political ruler who (very broadly speaking) used manipulation over persuasion and fear over love to ensure the loyalty of his subjects. These characters also drew heavily on the traditional Vice figure in English literature.
The Vice figure would use verbal cleverness to lead a protagonist into sin, using that protagonist's inherent moral weakness. Similarly, Lorenzo uses his verbal cleverness to lead the people around him to injustice, playing on their moral weakness as well as their lack of knowledge. And like the Vice figure, Lorenzo has a foil. In the morality plays, the foil was usually a virtuous old man. In this tragedy, the honest and virtuous Horatio acts as a foil. But a key difference between the Machiavellian villain and the Vice figure is that the villain is human, whereas Vice is supernatural (much like Revenge in this play). So Lorenzo is weak in the same way those he manipulates are weak, and he is as easily manipulated as those he manipulates. This ironic fact is proven by Hieronimo when he lures Lorenzo into the playlet, manipulating the young nobleman's love of theater and erroneous belief that Hieronimo bears him no hard feelings.