Non nobis solum nati sumus ortusque nostri partem patria vindicat, partem amici. (We are not born for ourselves alone; our country, our friends have a share in us.)
— Cicero, De Officiis
Marcus Tullius Cicero (sometimes called "Tully" in later references), 106-43 BC, was a prominent statesmen and the preeminent orator of late Republican Rome, as well as being single-handedly responsible for the persistence of the ancient Greek intellectual tradition in Western culture. Most people don't like Cicero anymore, which is a pity, because despite having issues with political consistency, he was absolutely brilliant and fought to the last for the Republic on the debate floor. During the wars following the death of Caesar he essentially gambled on his life and lost: he re-entered the political stage attempting to play Octavian and Mark Antony against each other, which failed when they ended up joining forces in the Second Triumvirate, and was proscribed by Antony in 43 BC. After evading his killers for some time with help from his many sympathizers, he was eventually betrayed later that year. Unable to escape or fight, he acted as a true philosopher and calmly offered his neck to the centurion's sword.
Not so much with Julius Caesar. They tended to be on different sides of a debate - especially during the civil war - but they shared a grudging respect for each other. Cicero was genuinely surprised by the assassination of Caesar.
Bilingual Bonus: His work is responsible for bringing many abstract philosophical terms from Greek into Latin, as well as for bringing abstract philosophy from Greece to Rome.
Courtroom Antics: Roman legal proceedings in this period were very nearly a spectator sport, and Cicero was practically an all-star quarterback. Antics Cicero got away with in court (copied from The Roman Republic page):
Accusing a political rival of incest in a completely unrelated case.
Pretending to be Rome personified (it's complicated).
Pretending to be a long dead Roman Consul (again, it's complicated). Incidentally, he was pretending to be the Consul so he could call the attempted murder victim above a shameless slut, whore, and murderess. And being quite graphic about it.
Complimenting opposing counsel on his skill — because the counsel was once one of his students.
Accusing opposing counsel of being gay.
Accusing the jury of being corrupt (although this was often the case).
Discussing fashion in the middle of a murder trial.
Discussing town planning in the middle of a murder trial.
Discussing highway maintenance in the middle of a murder trial.
Discussing the inconvenient placement of Public Holidays in the middle of a murder trial.
It is worth noting, after reading the above, that Cicero lost only one case. He lost that case because the court was filled with heavily armed, menacing looking men wanting a conviction and staring meaningfully at the jury throughout proceedings.
Compelling Voice: The leading orator of the ancient world, and arguably one of the greatest public speakers who has ever lived.
Dead Guy On Display: Antony had him murdered because Cicero made a series of speeches against him. His head and his hands (which had penned the speeches) were cut off and displayed in the Forum Romanum.
Deadpan Snarker: Notorious for being unable to keep his mouth shut. Octavian respected him, but didn't find it hard to let Mark Antony have his Roaring Rampage of Revenge upon him when Cicero joked about Octavian "We'll give him honors and decorations — and then a kick downstairs."
Downer Ending: Whether you like him or not, Cicero was assassinated for speaking out.
Graceful Loser: When Cicero's assassin came for him, he found him reclining on a litter reading a scroll. Cicero lowered the scroll to look at his soon-to-be-murderer and calmly said "Ah, there you are."
Heroic BSOD: Grieved heavily when his beloved daughter Tullia died after giving birth to her second son.
Historical Hero Upgrade: Traditionally, he was viewed very fondly by history (most likely because he was ultimately killed unjustly), even though, as demonstrated by the Long List of his Courtroom Antics, he had no regard for logic and reason at all, having practically invented the Chewbacca Defense, and was thus just as hostile to the idea of rationality and being unbiased as his opponents.
Perfectly Cromulent Word: Invented an awful lot of them. For instance, he is the reason "virtue" is a word. Of course, when you're trying to import a discipline with an entirely foreign vocabulary, you pretty much have to do this at some point.
"The Reason You Suck" Speech: One of the great masters of this trope. His orations against Catilina and the Philippics he wrote against Mark Antony are some of the best examples of this.
Self-Made Man: Famous for being a novus homo, lacking the significant lineage of 99% of politicians at the time. He came up through the Roman law courts and was the first member of his family to be elected consul, solely on his talent. Ironically, he caught a lot of flak about this from the entrenched patrician class whose traditional rights he often had to defend against (faux-)populist reformers like Catilina and Caesar.
Simple Country Lawyer: Part of his technique was the Roman version of this: like many novi homines, he came from one of the smaller towns in Latium (in his case, Arpinium) that had been granted Roman citizenship relatively recently. And he did work his way up the legal profession to Consulship and then the Senate.
After Antony fell to Octavian, the first Consul that Octavian appointed to serve with him was Cicero's son. It was Cicero the Younger who announced Marc Antony's death at Actium, and he later worked to remove Antony's entire family from political power, avenging his father's death with Augustus' approval.
Cicero in Fiction:
Appears in both seasons of HBO's Rome, played by David Bamber. In the series, the soldier who assassinates him turns out to be Titus Pullo.
Cicero turns up in Steven Saylor's Roma Sub Rosa series, employing protagonist Gordianus for various reasons, including to dig up information to help his defense of Sextus Roscius and to keep an eye on Catilina during the consular elections.
Colleen McCullough's Masters of Rome series follows Cicero's career along with many other prominent players in the late Republic.
Robert Harris' Imperium trilogy chronicles the life of Cicero from the perspective of his slave/scribe Tiro.
He briefly appears in Shakespeare's play Julius Caesar. The conspirators also consider including him, but Brutus rejects it, saying that Cicero won't support something that wasn't his idea.