refusing to give in

unyielding

(of a person) guided by one's sense of duty

thorough, careful

determined and stubborn when trying to solve or complete something

determined, persistent

stubborn

pig-headed, stubborn

decided and fixed; (of opinions, habits etc) fixed and not to be changed; unmoving; a group of games (in tennis etc); a group of similar things; the position (of the body, clothes etc)

decided

strong and determined; a loyal supporter

strong, valiant

not becoming less in force, degree

merciless