| relentless(adjective, appearance, mean, strong, hard) |
| staunch(adjective, strong, tough, firm, solid) |
| tireless(adjective, strong, firm, energetic, determined) |
| intractable(adjective, appearance, tough, firm, terrible) |
| steady(adjective, strong, firm, solid, determined) |
| unflagging(adjective, strong, firm, solid, energetic) |
| unrelenting(adjective, appearance, mean, hard, tough) |
| uncompromising(adjective, mean, hard, firm, harsh) |
| rigid(adjective, mean, hard, harsh, cruel) |
| unbending(adjective, mean, hard, tough, firm) |
| unfaltering(adjective, strong, firm, solid, energetic) |
| untiring(adjective, determined, persevering) |
| bullheaded(adjective, tough, firm, resist, determined) |
| insistent(adjective, determined, resolute, persistent, demanding) |
| perverse(adjective, mean, hard, tough, resist) |
| hard(adjective, mean, harsh, cruel, determined) |
| intransigent(adjective, tough, determined, resolute, stubborn) |
| resolved(adjective, strong, firm, deliberate, determined) |
| constant(adjective, strong, firm, determined, steady) |
| immovable(adjective, mean, hard, determined, fixed) |