Short answers, pulled from the story.
Gaius Cassius Longinus was a Roman senator and general who lived from around 86 BC to the 3rd of October, 42 BC. He is best known as the leading instigator of the plot to assassinate Julius Caesar on the 15th of March, 44 BC, and is described in ancient sources as the moving spirit of the conspiracy.
Cassius died by suicide after being defeated by Mark Antony at the Battle of Philippi in 42 BC. Unaware that his co-commander Brutus had won against Octavian the same day, he ordered his freeman Pindarus to help him die. Brutus mourned him as the Last of the Romans and buried him at Thassos.
Cassius served as quaestor under Marcus Licinius Crassus in the eastern campaign that led to the Battle of Carrhae in 53 BC. When the Roman army was crushed in what was considered the worst Roman defeat since Cannae in 216 BC, Cassius led the surviving troops back into Syria and organized the province's defence. Plutarch recorded that Crassus might have avoided the disaster by following Cassius's advice not to invade Parthia.
Cassius converted to Epicureanism sometime between 48 and 45 BC, a school of thought founded by Epicurus. Arnaldo Momigliano called this conversion a conspicuous date in the history of Roman Epicureanism and argued that Cassius used it not to enjoy private pleasures but to provide a philosophical justification for killing a tyrant. Scholars debate whether his beliefs actually motivated the assassination or merely offered a post-hoc framework for a decision driven by personal grievance and politics.
In Canto XXXIV of Dante's Inferno, Cassius is placed at the very centre of Hell, being chewed in one of the three mouths of Satan for eternity as punishment for killing Julius Caesar. He shares this fate with Brutus and Judas Iscariot, placing his betrayal of Caesar on the same moral level as Judas's betrayal of Jesus.
In Shakespeare's Julius Caesar, Cassius is a major figure as the leader of the conspiracy to assassinate Caesar. Caesar distrusts him and warns that Cassius has a lean and hungry look and thinks too much, calling such men dangerous. In the play's final scenes, Cassius tells a subordinate that the 3rd of October is his birthday, and he dies shortly afterward.