Cléante, the brother-in-law of Orgon, is portrayed as the voice of reason throughout the play. Siding himself with his sister, Elmire, he tries to convince Orgon of Tartuffe's hypocritical nature. Due to his stance against Tartuffe, he acts as a foil to the titular character, as well as Orgon. He is portrayed as a man who is well-spoken and wise, thinking himself a philosopher or sorts. This matches well with the story being conveyed, although his attempts at convincing his brother-in-law are moot.
As a foil to Orgon, Cléante displays certain aspects of his personality that are at an opposition to him. Where Orgon trusts Tartuffe without thought, Cléante is careful to think about what he sees. While Cléante uses logic and reason to try to convince Orgon of the truth of the matter, Orgon cannot process such logical thinking, relying on Tartuffe's word for his actions. As a foil to Tartuffe, Cléante speaks with truth and reason, but fails at the words of the hypocrit who uses narrative and reverse-psychology.