What important function does Jocasta play in the play Oedipus Rex other than being the mother/wife of Oedipus?
I think one of the most interesting aspects about Jocasta in this play is her approach to oracles and the way she tries to prevent Oedipus from discovering the truth that she is already beginning to suspect. Note how she is quick to insist that the Oracle that said Laius would be killed by his own son was false, as he was killed by highwaymen and his son had been disposed of. However, rather than reassuring Oedipus, these protestations begin to haunt him and he responds by asking for more details about the death of Laius.
It is clear that Jocasta in part is in denial and expresses this through her complete disbelief in the oracles in the play. Note how later on in the play, when a messenger arrives with news of the decease of Polybos, Jocasta appears hopeful that she can diminish the worry of Oedipus about fulfilling the prophecy. Note, significantly, how later on in this part of the play she delivers the memorable line: "May you never learn who you are!" as she tries to prevent Oedipus from questioning the messenger about his father. Before she hangs herself, in her last involvement in the play, she calls her husband/son "miserable." Her suicide and her obvious recognition of the truth points towards another tragic figure who nevertheless during her life operated to try and prevent the truth coming into light, denouncing and denying the veracity of oracles as suited her purpose.
In Oedipus Rex, Jocasta plays the following roles:
Peacemaker: she intercedes between Oedipus and Creon, trying to get the men to calm down, no doubt a testament to her mother-figure role.
Foil: she is a reflection of Oedipus. She triggers his memory and figures out the mystery of Lauis' murder sooner than he does. As such, she is a part of the way the dramatic irony functions.
Supplient: one who provides vision and unmitigated suffering and helplessness. She suffers only briefly in the tragedy, killing herself almost as soon as she realizes her crimes of murder and incest.
Not a tragic hero: Notice that she kills herself and Oedipus does not. Why? Her crimes were pre-meditated and more cruel: she tried to commit infanticide. Because of her suicide, Oedipus blinds himself with her pins, but he decides--unlike her--to take responsibility for his crimes. Thus, he becomes a tragic hero. She does not deserve the title because of her cruelty and lack of responsibility.