Bianca is Cassio's mistress in Shakespeare's Othello. Although Cassio is married (the only reference to this occurs at the beginning of the play, when Iago describes Cassio as "a fellow almost damned in a fair wife" (1.1.21)), audiences come to understand that he is having an affair with Bianca on the island of Cyprus.
With regard to the play's plot, Bianca functions to call Michael Cassio's credibility into question. Though Cassio is relatively respectful to Bianca, we see evidence that he doesn't take her seriously. As Othello watches from his hiding spot, he watches Iago question Cassio about Bianca (although Othello thinks they're talking about Desdemona), and Cassio laughs when Iago asks if he plans to marry Bianca. When she shows up, she throws Desdemona's handkerchief, which Iago had planed in Cassio's room, at Cassio, and Othello sees it.
Logistically, Cassio's plans with Bianca in Act 5 allow Iago and Roderigo to attack him, as they know he is having dinner with her late at night. After the plot to kill Cassio goes sour, Iago, in the presence of the Cyprus officials, accuses Bianca of being a whore and tries to implicate her in the attack. Obviously, though, the truth comes out at the end of the play.
Bianca is the mistress of Cassio, but is used by Shakespeare as more than just that. Her role is also important in the play and Iago used her in ruining the married of Othello. She is used for the comparison of jealousy. She becomes jealous after Cassio gives her a handkerchief, and this normal jealousy is compared against Othello's brutal, murderous, revenge seeking jealousy. When Iago brings Othello with him to prove Desdemona infidelity then Bianca appears in the scene and throws the handkerchief towards Cassio claiming that some other gave this to him and this proves his relationship with other woman. Also, in the eavesdropping scene, Iago jokes with Cassio about Bianca, but Othello thinks they're talking about Desdemona, sending him into a furious rage.
Bianca's entrance into the play is also significant because she is all that Othello is beginning to think of his wife (a whore). One is asked to compare Desdemona and Bianca - who are obviously contrasts. Desdemona is pure and faithful whereas Bianca is a courtesan.
Bianca is Cassio's lover. She gets upset because Cassio gave her a handkerchief, which belonged to Desdemona. Iago questions Cassio about marrying Bianca and Cassio says he loves her, which Othello mistakes as loving Desdemona. He then throws Desdemona's handkerchief up in the air, which enrages Othello even more.
I think Bianca is quite equally as important as Emilia and Desdemona. She clearly loves Cassio due to the Jealous rage she has when she questions him about the handkerchief. Your normal prostitute would not be bothered about that now would they? I feel sorry for her in a way as she just wanted to be with Cassio. I think she would have liked to have better treatment from Cassio as he was pretty nasty towards her. Overall though she seemed like a really nice person.