outline the relationship between Roderigo and Iago

Iago uses Roderigo rather than sees him as a friend. This line from Iago's soliloquy in Act I scene iii sums up his attitude to Roderigo -

Thus do I ever make my fool my purse:

For I mine own gain'd

Roderigo is used for sport in Act I scene i as Iago gets him to identify himself as the hooligan shouting in the street to Brabantio. It is of course Iago who is guilty of the most crude and vile comments, but Roderigo is is 'fool' and takes the blame.