Who is Captain William Bush from Lord Hornblower and what is their importance?

Captain William Bush is one of Hornblower's few close friends. They served together as lieutenants before Hornblower was appointed captain, whereupon Bush served as Hornblower's first lieutenant on numerous adventures. Because of Hornblower's influence and recommendation, Bush soon advanced to captain in his own right. As captain, Bush continued as Hornblower's right-hand-man after Hornblower's appointment as commodore. Bush is described as an imposing man with as powerful build.