steadfast

Synonyms: abiding adamant allegiant ardent bound changeless constant dedicated dependable enduring established faithful fast firm fixed immobile immovable inexorable inflexible intense intent liege never-failing obdurate persevering relentless reliable resolute rigid single-minded stable staunch stubborn sure tried-and-true true true-blue unbending unfaltering unflinching unmovable unqualified unquestioning unswerving unwavering unyielding wholehearted

Antonyms: disloyal unreliable untrustworthy