Bianca shows up in the play when everyone arrives on the isle of Cyprus. She is a courtesan attached to Cassio. In order to describe how she is "portrayed" in the play, we must look both at what she says and does and what others (mainly Cassio) say about her, the fact that she is described as a Courtesan or prostititue, being an influence on how she is percieved and treated by others.
Her first entrance into the play is in Act III, scene iv, and begins with tender conversation with Cassio. He calls her "most fair," "sweet love," and "sweet Bianca;" and she wonders why he has been absent from her house for "seven days and nights." They seem to have real affection for each other. And then Cassio produces the handkerchielf, given him by Iago. Bianca recognizes that it might mean that Cassio also has another woman. And at this, Cassio turns on her. Gone are his loving words, replaced by dismissive reprimand. They are both soon pacified, however, and in this scene they seem to be very affectionate with each other, and Bianca is portrayed as a loving, yet jealous, woman.
In Act IV, scene i, the situation is a bit different. Iago has decided to question Cassio about his feelings towards Bianca within the earshot of Othello, but by only referring to her as "she," Iago intends to have Othello mistakenly believe that they discuss Desdemona. In this scene, Cassio seems to do a 180 degree turn in his opinion of Bianca. He makes fun of her for loving him, and says:
I marry her? What? A customer?
I prithee, bear some charity to my wit,
Do not think it so unwholesome. Ha, ha ha.
Cassio basically portrays her as a common whore and his feelings about her only those of one of her "customers." Yet, he seemed to have real affection for her in the scene previous. Suddenly Bianca enters the scene, on the warpath. She has determined that he has given her some "minx's token," and she throws in in his face, saying, "[T]here, give it the hobby-horse, wheresoever you had it." And Cassio, all bluster and disdain only moments before is suddenly calling her "sweet Bianca" again, seemingly begging her to forgive him. She persists, however, telling him, in effect, that it makes no difference to her if he comes to see her again or not. And, like a chided boy, Cassio runs off after her.
The final scene with Bianca, Act V, scene i, shows her to be even more completely in love, and even more proud and unwilling to be put down. Cassio is wounded in the streets, and she, hearing the noise, comes out to find him, crying, "O dear Cassio, O my sweet Cassio." And, when Emilia and Iago cast aspersions on her at the end of the scene, she defends herself with pride:
I am no strumpet, but of life as honest
As you, that thus abuse me.
So, though we see Bianca for a total of three scenes in the play, she reveals her strength, her love for Cassio, and her honest, forthright pride of self. Cassio's opinion of her, however, is a bit more wishy-washy, as he seems to change his feelings as the breeze blows.
For more on Bianca and the scenes in which she appears, please follow the links below.