In the opening scene of Othello, Roderigo is upset that Iago has not shared a piece of recent news with him, even though Roderigo claims that they are close friends. Roderigo says of Iago that he "hast had my purse / As if the strings were thine." In other words, they are such good friends that Roderigo has given Iago his money to spend, but Roderigo believes Iago hasn't told him the latest news. In reality, Roderigo lusts after Othello's wife, Desdemona, and he pays Iago to help him get to Desdemona.
Roderigo says that he thought Iago hated Othello, and Iago says that he does. Iago responds that he does not know what is happening and explains that although several people recommended him to be Othello's lieutenant, Othello instead chooses Michael Cassio, who Iago believes is untested in battle. Iago believes that he is the right person for this position and that Cassio's only knowledge of battle comes from books. Roderigo urges Iago to quit, but Iago has grander plans. He will pretend to be loyal to Othello to serve his own ends.
In Act I of Othello, Roderigo complains to Iago that he is paying Iago money to get Desdemona to love and marry him, and--so far--it has not worked:
Roderigo is now poor and brideless. Brabantio has previously denied Roderigo as a suitor for his daughter, and now Roderigo cannot believe that Iago knows about Desdemona's elopement. You see, Roderigo is nothing short of a whiner, and he cannot believe that this beauty, the daughter of Venitian senator no less, would choose to secretly marry an older Black man over him.
Also, Roderigo will serve as Iago's mouthpiece as they both complain to Desdemona's father outside his home. Under cover of night, Roderigo will wake up the Senator and tell him that his daughter has eloped.