Othello: Roles of Cassio, Roderigo, and Brabantio, and their functions

The play 'Othello' by William Shakespeare is about three very important characters, Cassio, Roderigo and Brabantio, and their roles in the story.
Cassio is an honourable lieutenant and a soldier, who is
very loyal and caring towards his commanding officer, Othello, as shown
in one of many places, act two scene one line 45-48 when he is concerned
about Othello's well being because he has been lost at sea and says:
Thanks, you the valiant of this warlike isle, That so approve the Moor! O,
let the heavens Give him the defense against the elements, For I have lost
him on a dangerous sea
In…