Who is The Lord St. John from The Prince and the Pauper and what is their importance?

Lord St. John assists Lord Hertford as the Prince of Wales' guardian. He is appointed by the king after Tom and Edward switch identities, and the king thinks his son has gone mad. At one point, St. John suggests that Tom is not the prince, but Herford tells him that such an idea is treasonous. As a result, St. John quickly retracts.