Roderigo is a natural patsy. He is very gullible and easily manipulated by Iago. Iago makes great use of Roderigo's gullibility when he earns Roderigo's loyalty in stating a mutual dislike for Othello.
Roderigo was in love with Desdemona, but Brabantio forbade him to court her. He is devastated to learn that Desdemona had eloped with Othello. Iago uses this too his benefit. Iago is jealous of the power and position of Othello, and he finds it infuriating that he was not given the promotion that went to Cassius.
Iago prompts Roderigo to wake Brabantio with the news of Desdemona and Othello's departure. He is then able to implant insecurities and worries into Brabantio's mind about the dire consequences of Othello and Desdemona's relationship.