The word "tenacious" can be used to describe anyone who works very hard, despite the challenge or the level of difficulty. (Synonyms: persistent; determined; industrious; dogged)
Reggie is a very tenacious and diligent little boy. (diligent = hard working)
Thanks to some very tenacious employees, the company was able to pull through some tough times.
My dog is tenacious when it comes to playing fetch. He never wants to stop.
The word “tenacity” is a noun.
Thanks to the tenacity of the employees, the work was completed on time.
Motivation and tenacity will take you far in life.
What he lacks in knowledge, he makes up for in his tenacity.
The word “tenaciousness” is also a noun. It has same meaning as tenacity.