Who is Ligarius?

Caius Quintus Ligarius is one of the conspirators who participates in the assassination of Caesar in William's Shakespeare's play Julius Caesar. Ligarius is a character known to us through several ancient sources, including Plutarch and most importantly Cicero's speech "Pro Ligario" ("In Defense of Ligarius"). The basic history is that Ligarius supported Pompey in Africa against Caesar. Although Pompey lost the war, Caesar exiled Ligarius rather than having him killed. In a typically murky set of Roman political events, Caesar later pardoned Ligarius, allowing him to return to Rome and had him arrested. Cicero, the great Roman orator (who is mentioned in Shakespeare's play) volunteered to defend Ligarius.

The text we have of Cicero's speech Pro Ligario, relies almost exclusively on pathetic appeals for mercy rather than arguments about facts. Actually, the details of the case are barely mentioned. This suggest that Ligarius was indeed guilty, for Cicero's practice was to use facts when they were in the favor of his client and avoid them when the client was clearly guilty. As a result of being moved by Cicero's speech (and probably in an attempt to avoid immediate and direct conflict with Cicero at this time), Caesar pardoned Ligarius. 

In Shakespeare's play Ligarius is in uncertain health and somewhat reluctant to join the conspiracy, but eventually is persuaded to join by Brutus, and recovers his energy and sense of purpose.