Here're some differences and relationships between both:
In Othello, Shakespeare begins the play with two foils, Iago and Roderigo at night: one hidden and the other in plain sight. The former is a villain, the latter a fool, but both are be dark lords of misrule.
Here's what they have in common:
· Both are targeting Desdemona (Roderigo as thwarted love; Iago as revenge bait against Othello)
· Both are jealous of Othello
· Both are expose Desdemona's secret elopement to Brabantio (Roderigo publicly; Iago privately)
· Both lose their battle against Othello in Act I (the Duke condones the marriage)
· Both seek revenge in Cyprus (away from the Duke's protection)
· Both are id-based characters (exhibit child-like behavior and seek immediate pleasure: Roderigo=lust; Iago=suffering of others)
Here's how they are different:
· Roderigo is public in Venice and private in Cyprus; Iago is private in Venice and public and private in Cyprus: (as such, they are two halves of the hidden, dark side).
· Roderigo is pure passion (he threatens suicide); Iago is calculated passion (he wants only to wound others)
· Roderigo pays Iago. Roderigo is the "money" and Iago is the "purse."
· Roderigo is a fool. Iago admits, "I am not what I am" to Roderigo, and yet Roderigo continues to believe and pay him
· Roderigo thinks his using Iago for his dirty work, but Iago is really using Roderigo to be his scapegoat. Iago uses Rogerigo to expose Cassio.
Hope this helps!