In Othello, the courtesan who loves Cassio is Bianca.
Bianca appears briefly, but her relationship with Cassio becomes central to Iago's deception. Cassio asks her to copy a distinctive handkerchief that he has found in his lodgings. The handkerchief was originally Othello's gift to Desdemona, and Iago has planted it there to create false evidence of an affair.
Bianca is jealous because she thinks the handkerchief belongs to another woman. When she brings it back to Cassio, Othello watches the exchange from a distance and believes Bianca's jealousy proves that Cassio has been intimate with Desdemona. Iago has carefully arranged the scene so that an innocent conversation appears incriminating.
Bianca is sometimes simply described as Cassio's lover, while the play's characters also call her a prostitute or courtesan. She rejects that abuse, insisting that she is as honest as those who condemn her. Emilia is Iago's wife and Desdemona's attendant, not Cassio's lover; Desdemona is Othello's wife.