The most basic responsibility criminal lawyers have is to persuade the judge and/or prosecutor (sometimes a jury) that their client is innocent of the charges brought against them. Barring that, they can at least negotiate for a lighter sentence. Their first goal is to prove that their client is innocent or, at the very least, there isn't sufficient evidence for conviction. Their secondary goal is mitigation if their first goal can't be reached.
This can mean reducing felony charges to a misdemeanor, leading to a lighter sentence or even just a fine.
According to the Sixth Amendment of the United States Constitution, "In all criminal prosecutions, the accused shall enjoy the right to have the Assistance of Counsel for his defense." When a person in the United States is arrested, they are read their Miranda Rights, which include "You have the right to an attorney." On the whole, a criminal lawyer's job is to provide their client with their Constitutional right to a fair trial. Legally, the accused is presumed innocent until proven guilty. It's the criminal lawyer's job to see that their client is not proven guilty.
