Knave, loud sort, agitated very cool character (4,5)
I believe the answer is:
jack frost
'very cool character' is the definition.
(Jack Frost is a personification of winter)
'knave loud sort agitated' is the wordplay.
'knave' becomes 'jack' (synonyms).
'loud' becomes 'f' (short for forte - 'loud' in music).
'agitated' indicates anagramming the letters.
'sort' anagrammed gives 'rost'.
'jack'+'f'+'rost'='JACK FROST'
(Other definitions for jack frost that I've seen before include "nipper" , "Personification of winter weather that nips at your nose" , "Personification of a wintry morning" , "Personification of freezing weather" , "Cold weather, personified" .)