Foil: Literally a "leaf" of bright metal placed under a jewel to increase its brilliance (Holman & Harmon 198).
Traditionally, a foil is a minor character who, through comparison and contrast, serves to highlight the brilliance of the protagonist or hero. Iago is no minor character: he has more lines than the Moor. Iago is Janus, a foil unto himself, one who opens and closes scenes and the play itself.
If he is a foil of Othello, he's not a classical one anyway, and not my first pick. Iago is mainly a villain, and to call him otherwise, I think, subverts his purpose.
Othello is the gullible hero deceived by the melodramatic villain Iago. One critic's thesis of the domestic tragedy is "the wanton destruction of marriage by a villain." So in the morality play that is Othello, Iago is more like Satan to Adam and Eve (Othello and Desdemona): he's an entirely different creature with a singular, destructive purpose. It is this polar opposition that drives the play.
Roderigo and Cassio, white Christian Venetian males, are better foils for Iago. Publicly, Iago looks like them, talks like them, acts like them. Othello is on an island in this play, alienated by race, age, religion, rank, and status. The closest foil for Othello, I think, is the Turk, who ironically never appears, but lurks on the fringes. The Turk is Othello's "otherness," his Id, his jealous rage--hovering at the beginning and make flesh by the end.
The death of the Turkish forces at sea signals the beginning of the tragedy. This is when Othello's darker side, his morbid jealous as you say, begins to rear its head. Once on Cyprus, once the Turks have been defeated, once Othello lets his guard down, once Othello moves from a military man to a domestic civilian, Iago gains advantage and becomes the Vice figure who torments Othello, eliciting his self-loathing, fear, vaunted male pride, and morbid jealousy.