Strong-minded
determined, resolved, unwavering, steady, steadfast, firm, true, set, serious, decided, unshaken, persevering, unflagging, unfaltering, persistent, persisting, constant, set upon, intent upon, unchanging, bent on, immutable, fixed, settled, loyal, staunch, strong, stubborn, obstinate, faithful, knowing one's own mind, uncompromising, unyielding, strong-willed, tenacious, dogged, self-reliant, enduring, indefatigable, purposeful, single-minded, established, adamant, adamantine, inflexible, independent, irreconcilable, immovable, unshakable, intransigent, inveterate, obdurate, pertinacious, die-hard, gritting one's teeth*, out for blood*, putting one's foot down*, putting one's heart into*, having one's heart set on*, hellbent*.
Antonyms unsteady*, vacillating, wavering. See syn. study at faithful.faithful.