Who is Mr. Braun from Commodore Hornblower and what is their importance?

Mr. Braun is Hornblower's clerk and advisor. He was appointed by the Admiralty. Described as tall, spare, fair, young, and prematurely bald, Braun has pale green eyes, and he speaks Russian, Swedish, Finnish, Polish, German, French, and English with fluency. He also speaks Lithuanian and Estonian with passing familiarity. Braun considers himself a Finn, and he formerly fought and lost with Finland against the Russian invasion, after which, he escaped to Sweden where he is nominally a citizen.

Braun serves with capable loyalty until he goes ashore with Hornblower to attend a dinner party hosted by the Czar. When he discovers that his proximity to his hated foe will be assured, Braun steals one of Hornblower's rifled pistols and positions himself is a place where he will have a clear shot at the Czar upon entry. As Braun draws forth his pistol, Hornblower slices his hand open with a sword, an act that foils the assassination attempt and ruins Braun's future. Braun is then restrained by Hornblower's crew and taken back to the ship, where he languishes in isolation. Hornblower ships Braun off to England at the earliest possible opportunity, where he is tried for treason, found guilty, and hung. Braun's betrayal of trust is heavily foreshadowed in the novel and is not particularly surprising.