In Sophocles' Oedipus the King, Creon is the brother of Jocasta, which makes him the brother-in-law of Oedipus.
In the first part of Sophocles' play, Creon serves as sort of an ambassador for Oedipus, as Creon goes to the Delphic oracle to learn what the Thebans should do to remedy the plague that afflicts their town.
Creon's next appearance has a much different tone as he tries to defend himself from Oedipus' accusation that he is plotting against Oedipus. Ultimately, the argument between the two men becomes so heated that Jocasta has to intervene.
By the end of the play, after Oedipus blinds himself, Creon becomes king of Thebes and Oedipus begs him to take care of his two daughters. As at the first of the play, Creon consulted the Delphic oracle, now too at the end of the play Creon indicates, in response to Oedipus' request to cast him out of Thebes, that he will make sure this is what the gods intend:
That is something I could do, of course,
but first I wish to know what the god says
about what I should do. (Ian Johnston translation)
Thus, in Sophocles' play, Creon appears as a loyal and god-fearing person who does his brother-in-law's bidding in the first part of the play, then suffers his accusations of treason, and finally treats him with some compassion at the play's conclusion.