Cassio is noble minded and loving toward Desdemona and Iago throughout. These trusting and noble traits and opinions of Cassio's are in part what allows Iago to succeed in his vengeful plan. In short, throughout Acts I, II and III, Cassio has the highest opinions of both Desdemona and Iago. His opinions about these two never change until Lodovico reveals some of Iago's intrigues exposed in the letters "in the pocket of the slain Roderigo."
Sir, you shall understand what hath befall'n,
Which, as I think, you know not. Here is a letter
Found in the pocket of the slain Roderigo;
And here another: the one of them imports
The death of Cassio to be undertook
The opinion Cassio holds of Desdemona is that she is a good, faithful, loving and noble woman, worthy of his general, Othello. Cassio calls her "our captain's captain" to signify how thoroughly she and Othello have bonded and how deeply Desdemona has won Othello's love. When Cassio is in the depths of Iago's plot against him and Othello, he accepts Iago's advise to plead his cause with Desdemona in hopes that she might speak with Othello on Cassio's behalf to try to have Cassio vindicated and reinstated. Cassio accepts Iago's advise to act thus because he believes in Desdemona's goodness and ability to sway Othello with right thinking. Thus Cassio encompasses in his opinion an elevated idea of Desdemona's inner character traits and of her intellectual traits, for he must trust her ability to reason and persuade if he is to trust her ability to influence Othello.
The opinion Cassio holds of Iago is that he is a officer worthy of his service to Othello. Cassio has no hint of Iago's smoldering hatred for Othello nor of his dark suspicions about Othello's conduct with Emilia.
I do suspect the lusty Moor
Hath leap'd into my seat; the thought whereof
Doth, like a poisonous mineral, gnaw my inwards;
And nothing can or shall content my soul
Till I am even'd with him, wife for wife,
Cassio further has no hint of Iago's plan of revenge that hinges upon the destruction of Cassio and Othello, together. Cassio knows the outer character traits of Iago without having had a glimpse of the inner traits of hatred and revenge brewing in Iago's dark heart. Cassio further has no inkling that Iago could be the sort--after destroying Othello's, Desdemona's and Cassio's lives--to drive his sword through his own wife, Emilia, killing her, after she exposed his villainy, in order to make his own escape.
[Desdemona] give it Cassio! no, alas! I found it,
And I did give't my husband.
Filth, thou liest!
By heaven, I do not, I do not, gentlemen.
O murderous coxcomb! what should such a fool
Do with so good a woman?
Are there no stones in heaven
But what serve for the thunder?--Precious villain!
[Othello] runs at IAGO. IAGO, from behind, stabs EMILIA, and exits.
Cassio holds an opinion of Iago--right up to the end--that elevates Iago to a level of goodness and noble behavior making him worthy of being the officer of the great Moor general Othello.