firm and unwavering
steadfast - firm
If somebody is resolute and determined with regards to a belief or desire, or fiercely loyal or supportive to a person or cause, then they can be described as ‘steadfast’. This word implies strength of character and trustworthiness rather than stubbornness, so it is almost always used to express approval.
I can’t fault the way my family has backed me during this difficult time. They have been totally steadfast in their support.
To succeed in pushing through the reforms we want, we must remain steadfastly resolute in the face of intense opposition.
Peter’s lack of steadfastness concerns me. I do not think that he is capable of performing under pressure.