Sebastian is the name of Viola's twin brother in William Shakespeare's Twelfth Night. When Viola first washes ashore in Illyria following a shipwreck, she believes her brother to have died at sea and assumes a male disguise so that she can find employment with the Duke Orsino, taking on the name Cesario.
In act 2, scene 1, the audience learns that Sebastian has not drowned but has in fact also arrived in Illyria; he assumes Viola is dead. The uncanny resemblance between Sebastian and Viola in her "Cesario" disguise leads to many cases of mistaken identity as the play progresses.
It is not until act 5, the very end of the play, that the twins are reunited. The revelation that the person everyone had assumed to be Cesario was in fact two people—disguised Viola and Sebastian—allows for the complicated love triangle of the play to resolve itself.