She is willing to help Cassio and attempts to speak with Othello about Cassio.
She doesn't understand why Iago wanted Desdemona's handkerchief, but when she has the opportunity to get it, she does give it to Iago.

He believes that Iago is a loyal friend, and he trusts Iago completely.
If we used the term "brainwashed", that would accurately describe Othello. He believes all of Iago's lies. He asks Iago for physical proof of the affair, infidelity.
He could best be described as conniving. He is a two-faced liar.

He appears to the characters in the play as their friend, but through his soliloquys, the audience knows that he is plotting revenge and telling lies.

Relationship of Cassio, Desdemona, and Emilia
The three people work together to create a plan to speak with Othello and persuade him to reconsider Cassio's demotion. The idea to do this was originally Iago's. Othello will become suspicious of Desdemona's interest in speaking of Cassio.
Relationship of Iago and Othello
Iago begins to tell Othello that he suspects that Desdemona is really in love with "her own kind" --a white man, from Venice.

She is really in love with Cassio, and her love for Othello is a "passing fancy". Othello believes Iago's lies.

Iago's conversation with Othello
In order to make Othello believe that Cassio and Desdemona are having an affair, he makes up the story of Cassio's dream. Iago tells Othello that one time, when he had to share a room with Cassio, Cassio cried out Desdemona's name in his sleep, and he thought that Iago was Desdemona and he tried to kiss Iago. Othello is devastated.

Relationship between Desdemona and Othello
Othello is suspicious of Desdemona's intentions in clearing Casio's name. He believes Iago's lies, and he now looks at Desdemona with suspicion. Desdemona is unaware of Othello's thoughts.
Strawberry Handkerchief
This is.

the first gift Othello gave to Desdemona. It was Othello's mother's handkerchief. Supposedly, it has magical powers, and as long as the person who was given the handkerchief has it, he or she will remain in love with the person who gave it to him or her. When Desdemona accidently drops the handkerchief, Emilia picks it up and gives it to Iago.

Othello's behavior
We see a change in Othello's behavior. He is angry, for the first time, at Desdemona.

She is confused by his behavior. Iago is delighted by the change in behavior--it means his plan is working, and Othello believes his lies.