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.
Caius Ligarius is one of the conspirators against Caesar. He doesn't like Caesar because he was embarrassed by Caesar for Ligarius's positive statements about Pompey. In Act II, Metellus tells Brutus:
"Caius Ligarius doesn’t like Caesar,Who berated him for speaking well of Pompey. I wonder why none of you’ve thought about him. And Brutus responds, "Well, ask him, he likes me very well as I have given him reason to. Just send him to me and I will train him."
Then in later in scene one, Caius Ligarius, arrives at the home of Brutus as a weaken and sick man. Brutus tells him that he is sorry that he is sick. Ligarius tells Brutus that because of his respect for Brutus and his dislike for Caesar he is not too sick to help.