| stringent(adjective, severe, harsh, cruel, determined) |
| strong-minded(adjective, determined, fixed, stubborn, hardheaded) |
| strong-willed(adjective, firm, intense, deliberate, determined) |
| sure(adjective, strong, firm, solid, certain) |
| telelogical(adjective, intense, deliberate, determined, resolute) |
| tenacious(adjective, strong, intense, deliberate, determined) |
| tireless |
| true(adjective, strong, firm, certain, definite) |
| unalterable(adjective, strong, definite, fixed, impassive) |
| unappeasable(adjective, harsh, cruel, determined, fierce) |
| unchangeable(adjective, strong, certain, steady, definite) |
| undaunted |
| unfaltering(adjective, intense, energetic, deliberate, determined) |
| unflagging |
| unflappable(adjective, set, firm, solid, determined) |
| unforgiving(adjective, harsh, cruel, determined, fierce) |
| unqualified(adjective, strong, firm, solid, certain) |
| unrelenting(adjective, harsh, cruel, determined, fierce) |
| unremitting |
| unshakable |