Sebastian is one of the main characters from William Shakespeare's play Twelfth Night, believed to have been written around 1600 or 1601.

After the beginning of the play, Viola mentions that her brother Sebastian is drowned in the sea.

After three months with Antonio, Sebastian decides to set out to Illyria, ruled by Duke Orsino.

Meanwhile, Viola has disguised herself as a male page named Cesario and works for the Duke.

Because they look so much alike, when Olivia sees Sebastian later, after he has not backed down from a challenge by Sir Andrew Aguecheek, she thinks he is "Cesario".