steadfast

Synonyms:
2. sure, dependable, reliable, constant, unwavering. Steadfast, staunch, steady imply a sureness and continuousness that may be depended upon. Steadfast literally means fixed in place, but is chiefly used figuratively to indicate undeviating constancy or resolution: steadfast in one's faith. Staunch literally means watertight, as of a vessel, and therefore strong and firm; figuratively, it is used of loyal support that will endure strain: a staunch advocate of free trade. Literally, steady is applied to that which is relatively firm in position or continuous in movement or duration: a steady flow; figuratively, it implies sober regularity or persistence: a steady worker. 4, 5. stable.

Antonyms:
2. capricious, variable.