They are the guards in Hyrule castle (they also serve military purposes) there are Many ranks.
they are the most common however they are mostly cowardly only a small number are actually brave.
they are Far less cowardly than there underlings and wield long swords they are unusally tall.
Royal Guards are the best of the best they guard hyrule castle from intruders.(Note:like guard captains they too are unusually tall.)
there are rumors that the reason why guard captains and Royal guards are so tall is because they are magically enhanced.(either that or its the boots.)